Si B 

731 I 

Wfc5 mS in HEALTH and DISEASE 



being 



An Abstract of a Course of Lectures 
delivered in the University of Manchester during the cession 



BY 

F. E. WEISS, D.Sc, 

Harrison Professor of Botany, 

A. D. IMMS, M.A., D.Sc, 

Reader in Agricultural Entomology^ 

AND 

WILFRID ROBINSON, M.Sc, 

Lecturer in Economic Botany, 



Manchester 
AT THE UNIVERSITY PRESS 
12, Lime Grove, Oxford Road. 

LONGMANS, GREEN & CO. 
London, New York, Bombay, 
etc. 

1916. 

1/6 Net. 



PLANTS IN HEALTH AND DISEASE. 



Published by the University of Manchester at 
THE UNIVERSITY PRESS 
(H. M. McKechnip:.. Secretary), 
12, Lime Grove, Oxford Road, Manchester, 

LONGMANS, GREEN & CO. 
London : 39, Paternoster Row. 
New York: 443—449, Fourth 

Avenue and Thirtieth Street. 
Bombay : 8, Hornby Road. 
Calcutta : 303, Bowbazar Street. 
Madras : 167, Mount Road, 



PLANTS in HEALTH and DISEASE 



being 



An Abstract of a Course of Lectures 
delivered in the University of Ma7ichester during the sessio?i 

1913-16 



, jt.* - BY 

F. E. WEISS, D.Sc, 

Harrison Professor of Botany, 

A. D. IMMS, M.A., D.Sc, 
Reader in Agricultural Entomology^ 

AND 

WILFRID ROBINSON, M.Sc, 

Lecturer in Economic Botany. 



Manchester 
AT THE UNIVERSITY PRESS 
12, Lime Grove, Oxford Road. 

LONGMANS, GREEN & CO. 
London, New York, Bombay, 
etc. 

1916. 



PREFACE. 



The cour-e of lectures of which thi5 little book is a summary 
was undertaken vnth a view to giving -ome assistance to those 
who were endeavouring, to the best of their ability, during the 
present crisis to increase the productiveness of their gardens or 
allotments. V\ as it po-sible for the L'niversity to aid in any 
way the-e practical men and Avomen to dccomplish their laudable 
and patriotic endeavour ? A kno~\A ledge of the structure and 
life of the plants they cultivate could not fail to be helpful to 
gardeners and allotment-holder- in exphnning the reasons for 
many of the common horticultural practices. Familiarity also 
with the common animal ,tnd fungal para-ites of our garden 
crops, and the method- of combating these pest- Avould enable 
them to saA-e many doomed plants. For these reason- this course 
of lectures on •' Plant- m Health and Disease "* v.a^ m-tituted, 
and. as the size of the audience indicated that, the lectures met 
a real need, it seemed desirable to is=ue to members of the 
audience a short eight-page summary of each of the lectures. 
As we ha\"e received man}' enquiries from person^ not attend- 
ing the lectures both for .-mgle ai^stracts. and -mce the con- 
clusion of the cour-e. fur comi^lete -et-. we have decided to 
reprint them m book form. W > tra-t. hoAvever. that the fact 
Aviil not be ox'crlooked that ihi- i-.-ue does not pretend to be 
more than a somevhat brief -umrnary of a course of lectures, 
and that all the lecturer- wt-ie tied down to very narrow limits 
wherein to compress the suoject matter of a much longer dis- 
course. We would also iDomt out that, as the lectures vcere 
addressed to a ]\Ianchester audience, the lecturers often dealt 
with the peculiar dihiculties that cire met with in this neighbour- 
hood, and that the accounts given uf the animal and fungal pests 
do not profes- to be exhaustive, but are descriptn-e of the niore 
common dlsease^ occurring in the gardens and allotments m the 
vicinity of our large industrial tuvuis. i he_ necessary condens<i_- 
tion of many interesting point-. Avhich might wiih cidvant.igt- 
have been expanded, and the omission of the illustrations which 
accompanied the lecture-. wiU probably be less noticeable to 
those Avho have attended the course of lectures than to new 
readers of this little volume: nevertheless, we hope that these 
latter will <ilso nnd in it -ome information, which may be of 
value to them, bhould this hope be realised we shall feel well 
satisfied. 

Thf. Umversity, F. E. Weiss. 

Waxchester, a. D. Imms. 

i;th March. 1016. W. RoBixsox. 



TABLE OF CONTENTS. 



PAGE 

Chapter r. F . E. Weiss i 

General Featl'res of Plant Life. 

Xutrition <aid propagation, vegetative and seed reproduc- 
tion. Annuals, biennials, and perennials. General account 
of vegetative orgnns. 

Chapter 2. F . E. Weiss q 

Roots and Root Xutrition. 

Absorption by roots and root pressure. Physical and 
chemical nature of soil. ^Manures and their importance. 
i3cictcria m ^oil. Root tubercles of Leguminous Plants. 
Rotation of crops. Trenching and ridging. 

Chapter 3. F . E. Weiss 17 

Stl:.is and Leaves. 

]\Iechan]cal r ecpjircnients of -terns and branches. Twin- 
ing and climbing iDlants. The food conducting function of 
stems. The Avork of leaves and their structure. The wilting 
and recovery of leaves. Protection of leaves against exces- 
sive drought. Direct eft'ect of surroundings on leaf develop- 
ment. Hardening otf plants. 

Chapter 4. F. E. Weiss 25 

Methods of Vegetative Reproduction and Propagation. 

Tubci's. liull;- and corms. Bulbils. Runners. Layer- 
ings and cuiiings. Budding and grafting. 

Chapter 5. F. E. Weiss 33 

Flowers an'd their Formation. 
Conditions la\'ouring the production of ilowers. Structure 
and functions of the wirious parts of a liower. Pollination 
and Fertilisation. Self-fertile and self-sterile flowers. 
Ripening of fruits and seeds. 

Chapter 6. F. E. Weiss 41 

Seeds an"d Seedlings. 

Vitality and longevity of seeds. Conditions favouring 
germination. Seedlings. \'ariation. Natural and artificial 
selection. Sports or mutations. Plybrids and the laws of 
heredity. 



vii 

PAGE 

Chapter 7. F. E. W eiss 40 

Malformations and Injuries. 

Alalformations arising as sports or by malnutrition. 
Healing of wounds. Injuries due to lightning, frost, etc. 
Harmful effect of smoke and fog. 

Chapter S. Wilfrid Robinson 57 

Fungi as a Cause of Disease in Plants. 

Common fungi, mushrooms, toadstools and moulds. How 
they live. Saprophytes and Parasites. The " Damping-off 
disease of Seedlings caused b\' the fungus Pythium. Life 
Story, means of spreading and living in the soil, pre- 
cautionary measures. 

Chapter q. Vi'ilfrid R ohms on 65 

Diseases of Roots and Tfbfrs. 

Fingers-and-Toes of Turnips, Cabbages, etc. Habits and 
life history of the fungus. Remedial treatment. " Limmg 
01 the soil. etc. Similar diseases, e.g.^ Black Scab or Wart 
Disease of Potatoes, *' Spongy Scab," etc. 

Chapter to. IVilfnd Rob in. son 73 

A Disease of Leaves, Shoots and Tui^>ers. 

The Late-Blight disease of potatoes. Symi^toms and 
means of spreading. Diseased tubers and the wintering of 
the fungus. Treatment of the disease, sprayiiig and the use 
of resistant varieties. 

Chapter ii. Wilfrid Robinson 81 

Diseases of Leaves and Shoots i.contd.). 

Black-Leg or Wilt Disease of Asters. Tomato-leaf rust 
or mould. Leaf-b]otch of Cucumber. ]\Iildews of Roses, 
Gooseberries, etc. 

Chapter 12. Wilfrid Robinson 8g 

Diseases of Leaves (contd.). 

Rusts. The ^\'heat Rust. Life Story of the fungus, 
relation to the Barberry. The Mint Rust and Hollyhock 
Rust as examples of other Rust diseases. 

Chatter 13. A. D. Imms 07 

Injurious Animals other than Insects. 

Injurious and beneficial Animals. Birds and the need 
for scientific investigation Avith reference to their food at 
different seasons of the year. Eelworms and more especially 
the Stem Eehvorm and the Knot Root Eelworm. Pul- 
monate Alolluscs (Snails and Slugs). The Black Currant 
Gall Mite. 



Vlll 



Chapter 14. . ^^age: 

C Ia-jurious Insects ^ ' 

Chapter ,5. 4 n ; 

Chapter iG. j r z 

rests and J- umigation Methods. ^n^ect;. Greenhouse 

Chaptkr 77. in/ 
Bn;. Th T ETC. ""-^ 

Beneficial Insert,.'' The"f.ie'''J,,--^;!"^'"-"™S- Centipedes 
ture bear.ng upon Econoi!iic Z^olo^i?' '''''''''^ 



Index 



137 



Plants in Health and Disease. 



Chapter 1. 

GENERAL FEATURES OF PLANT LIFE, 

Nutrition and propagation^ vegetative- and seed reproduction. 
Annuals, biennials, and perennials. General account of vegeta- 
tive organs. 

Gardeners being concerned m the cultivation of plants, 
it IS obviously important that they should be acquainted 
with their structure and mode of life. ihey will find it 
particularly useful to know in what ways plants respond 
to their environment, for such knowledge will enable them 
by varying their treatment to modify the development of 
the plants they cultivate, to accelerate or retard their 
growth and to ensure the production of a greater number of 
flowers. The gardener's aim is not always the same. The 
flow^er lover is anxious to obtain a wealth of bloom. The 
allotment holder on the other hand, concerned in raising 
vegetables, may desire well-developed leaves, roots or 
tubers. It is important therefore that we should study 
both ^ke vegetative and the refroditctive organs. 

The former are concerned mainly with nutrition, the 
latter with the propagation of the plant. These two sets 
of organs are often differently affected by external con- 
ditions. A gardener knows that by over-stimulating the 



I 



2 



development of foliage he may be endangering the rym- 
duction of flow ers. It IS a common practice to reduce rlie 
supply of water so as to encourage the formation of fiovs er 
buds. Some of our common British fruit trees when grown 
in tropical climes will develop into luxuriant trees with a 
wealth of foliage, but lack the flowers essential for the 
production of fruit. The practice of pruning and root 
pruning is based on the same phenomenon, and b\' reducing 
the amount of vegetative organs the formation of flower 
buds is encouraged. Yet though there seems to be this 
opposition between vegetative and reproductive organs the 
latter are really dependent upon the former. For it is 
only at the expense of the food material absorbed and 
worked up by the roots and leaves that the flowers are 
produced. The activity of the vegetative organs must 
therefore always precede the formation of flowers. 

This is seen very clearly in the case of annuals. These 
plants complete the whole of their life-cycle m one \-egeta- 
tive season which is usually much shorter than a year, m 
some arid regions amounting only to a few weeks during 
and immediately after the wet season. An annual coiPx- 
mences as a seedling at the beginning of the favourable 
vegetative season, and after the production of a limited 
number of leaves produces its flowers, which are kept sup- 
plied with food material by the activities of roots and 
leaves until the seeds have matured, then the whole plant 
dies dowm to be replaced next season by the offspnng 
developed from its seeds. Such is the life history of 
the mustard and cress and also that of most of our com- 
mon weeds like chickweed, groundsel, and some of our 
grasses. The ubiquity of our weeds is clue not oiikr to 
their effective means of dispersal, but also to their rapid 
growth to maturity which enables them to produce two 
and even three generations of plants in one season. 

Slower in their development and exhibiting a m.ore 
marked contrast of vegetative and reproductive periods 
are the so-called bienjiials. In these plants after the vege- 
tative organs are produced the}' are emplo}'ecl throughout 
the first summer season in manufacturing and storing a 
large supply of food material, which is to be used in the 
formation of flowers and seed during the second year, 
after which the entire plant dies down having fulfilled 
its existence and produced a vast number of oft'spnng. 
For the large store of food material which it has laid up 



during the first year will enable it to produce a much 
larger number of flowers than is commonly the case in 
annual plants. In most biennials the food material is 
stored in some underground part of the plant such as the 
swollen ' root ' of the turnip or beet, the leaves during the 
first season forming a tuft or rosette close to the ground. 

Lastly we have perennials which are much more varied 
in character than the first two groups of plants. These 
latter are always comparatively soft m texture, but 
perennials include both herbaceous and woody forms 
such as trees and shrubs. The herbaceous again are of 
two types, firstly those w^hich persist throughout the winter 
like violets and primroses, and secondly those which die 
down in the autumn leaving a persistent root or root- 
stock underground, from which the plant renews its 
growth in spring. Plants of this kind like the iris, peony, 
larkspur, Michaelmas daisy and many other favourites 
of our herbaceous borders have like biennials a large store 
of food material in their underground organs. This 
enables them in most instances, not only to produce 
annually a crop of flowers but to branch out underground 
and develop into ever-spreading clumps, which in many 
cases require repeated breaking up and thinning just as we 
require to cut back our bushes and trees. In some cases 
these underground portions of perennial plants do not 
remain attached m one mass, but when the plant dies down, 
a good deal of the underground part dies away too, leav- 
ing isolated portions, so that in place of one individual 
we find many fragments which would seem to be offspring 
though they are really only remnants of the original 
parent. Such offsets we have in the case of the tubers of 
the potato, which represent the rounded swollen ends of 
underground shoots that have become entirely separated 
one from the other. Though this is really only a breaking 
up of the original plant, it is often spoken of as vegetative 
refroduction but must not be confused with seed refroduc- 
tion, which is always the result of the fertilisation of 
flowers. It is important to differentiate these two methods 
of propagation, particularly as in the case of the potato 
the tubers used for setting in spring are termed "seed 
potatoes," though they have really nothing to do with 
seeds. Vegetative reproduction does not replace seed re- 
production but is an additional means of propagation, 
often of the greatest use both in nature and in cultivation. 



4 



The potato, for instance, though it produces flow-ers in 
this country very rarely contrives to ripen its seeds in our 
cHmate and can only be propagated in England by its 
tubers, which are indeed the sole reason for its cultivation, 
for these tubers richly stored with food material are of 
the greatest importance as a staple food of mankind. The 
Jerusalem artichoke has the same way of vegetative repro- 
duction and a very similar process obtains in all bulbous 
plants. 

Most perennial plants produce a crop of flowers and 
fruit every season, in some cases after a shorter or longer 
period of immaturity as is usual for instance in shrubs 
and trees. These flowers may make their appearance in 
spring before the foliage. But in that case the flowers are 
produced at the expense of the food material built up by 
the leaves of the preceding summer, while the fruits are 
generally matured by the activity of the leaves of the samie 
season. 

Let us now consider some of the effects of external 
factors upon the growth of plants. If we germinate a seed 
under suitable conditions on the surface of the soil we 
And that when the young root breaks through the seed- 
coat it bends downward and penetrates into the earth. 
That this is due to the effect exerted by gravity on the 
young growing root can be demonstrated by slowly rotat- 
ing a growing plant on a horizontal axis, when it will be 
found that the root will grow out horizontally as gravity 
acts first on one side of the root and then on the other, and 
thus its effect is eliminated and the root is not affected. 
The main stem of a plant is equally sensitive to the force 
of gravity, but responds in a different manner growing 
in the opposite direction to it and, if laid down horizon- 
tally, bending upwards at right angles. 

Detailed microscopic examination has shown that 
plants have special regions of perception and it was found 
by Darwin that as regards its sensitiveness to gravity the 
seat of perception was the root-tip. and that if this was 
cut off the root ceased to respond to gravity. In the stem 
it is known that the perceptive region is not so limited in 
extent. But while the main root and the main stem tend 
by their response to gravity to grow in a vertical direction^ 
lateral roots and lateral branches do not respond in a 
similar manner but tend to place themselves more or less 
horizontally or obliquely. A most curious feature of plant 



5 



life, and one very difficult of explanation, is the fact that 
when the tap root or the mam stem of a plant is destroyed, 
a lateral branch will take its place and assume the vertical 
position. The importance of this position is self evident 
when we consider the functions of the root and of the 
stem. The former acts as the absorbing organ, extracting 
from the soil the water and valuable salts necessary for 
the growth of the plant; it is obviously essential therefore 
that the root should grow downwards in search of moisture. 
It is also important that the lateral roots should not grow 
in the same direction as the mam root, so that they can 
search out other regions of the soil in their quest for food 
material, ihe spreading habit has the further advantage 
that it anchors the plant more firmly m the soil and pre- 
vents it from being easily uprooted by the wind. 

The stem on the other hand growing away from the 
soil is in an advantageous position for exposing the leaves 
it bears to the full rays of the sun and thus enabling them 
to fulfil their main function in the life of the plant. This 
function is to absorb as much light as possible and by con- 
verting the light rays into energy to build up the organic 
material upon which the formation of flowers and fruits 
depends. Light being therefore of such vital importance 
to plants we find that in the course of their evolution they 
have acquired the power of responding in their growth to 
the stimulus of light. It is a familiar fact that stems of 
plants will bend towards the light if instead of being 
illuminated from all sides they receive the light from one 
side only. Growing in the open the main stem is erect 
and the leaves are usually expanded horizontally, that is 
at right angles to the direction of maximum illumination 
which is from above. When shaded on one side the stem 
inclines towards the light and the leaves are placed 
obliquely, often indeed horizontally, when a plant is grown 
in a window, in which case it is more or less shaded on 
three sides. Roots as they are underground do not nor- 
mally exhibit in their growth any response to light, but if 
a young seedling plant is grown with its roots in a glass 
jar it will be found that if illuminated from one side only 
the roots will bend away from the light. In nature this 
can be observed in the roots which grow out from the stem 
of the ivy when clinging to a wall. They are generally 
produced from the shaded side of the stem and always 
bend away from the light and towards the wall. These 



6 



climbing roots are not sensitive to gravity as they have 
a very definite function to perform in fixing the plant to 
the wall, and are not concerned in the absorption of food 
material. 

Wherever bending, such as described above, takes place 
in growing organs, this is due to differences in the amount 
of growth on different sides of the stem or root. The con- 
cave side grows less than the convex side. Thus when a 
stem, which has been laid horizontally, bends upwards, 
this is due to the greater amount of growth of the side 
nearer the ground. If a stem illuminated from one side 
bends permanently in that direction this is due to the fact 
that light retards the rate of growth and the side away 
from the light growing more rapidly the stem becomes 
convex on this side and bends tow^ards the light. The 
fact that light retards growth and therefore causes plants 
to be short and " stocky " is of course a well-knowTi 
phenomenon, while the lack of illumination acting like 
darkness causes more rapid growth and we get long 
" leggy " plants, when they are insufficiently lighted as 
when grown in the shade, in deep frames or pits, or not 
close up to the lights in greenhouses. 

There are one or two other factors influencing the 
growth of plants which it may be useful to refer to at this 
juncture. Besides being sensitive to gravity roots are 
also sensitive to contact, and when a root tip comes in 
touch with a solid body such as a stone in the soil it bends 
away from it. This is brought about as in the case of 
other bending movements, by the fact that contact causes 
retardation of growth on this side of the root and this side 
becomes therefore convex, the root-tip pointing away from 
the obstacle met with in the soil. In this way it is possible 
for the root to make its way even through a stony soil, 
avoiding or rather growing round all obstacles with which 
it may be met, bending to right or to left in its progress 
downwards. This sensitiveness to contact is however not 
only possessed by main roots which grow dowTiwards ; it 
is equally important to lateral roots. 

It has been found in the somewhat infrequent cases 
occurring in nature that where the soil is drier on one side 
of a plant than on the other the root system develops 
more abundantly in the moister soil. Experimentally, too, 
we can prove that a root will grow towards moisture just 
as the stem of a plant will grow towards the light. 



7 



Lastly, we have plenty of evidence thai m addition to 
their need for water the roots of plants, just like the living- 
parts of all plants or animals require for their growth that 
life-giving constituent of the air, which we also breathe 
in, namely, oxygen. This need of the roots for air is one 
of the fundamental facts which governs not only the dis- 
tribution of plants in nature, but dominates our whole 
agricultural and horticultural practice. It is this need for 
air which causes the farmer to drain his fields and the 
gardener to grow his plants in porous and well-drained 
pots. If we turn a plant out of such a pot we shall see 
by the dense felting of the roots in close contact with the 
sides of the porous pot and among the crocks at the bottom 
of the pot how eager the roots arc for air. If on the other 
hand we do not secure drainage or if we over-w^ater 
pot-plants we soon find that they show signs of ill-health. 
As a matter of fact their roots are being suffocated and 
may die away unless we alter our treatm^ent. Probably 
more plants are lost by over-watering than by insufficiency 
of water. Normally, therefore, though water is a prime 
necessity of plants its provision must not interfere with 
the respiratory process of plants, and w^e must always 
provide a porous soil for our crops, that is a soil with 
sufficient air. This is one of the reasons for the practice 
of hoeing. For apart from the clearing out of weeds, 
hoeing prevents the ground from becoming caked, a con- 
dition w^hich would prevent the free access of air into the 
soil. At certain stages in their growth this need seems 
greater than at others. Speaking generally we may say 
the more actively growth is proceeding, the greater the 
need for air. Germinating seeds for instance require a 
large amount of air, and when the formation of new roots 
IS proceeding in the case of cuttings and layerings a porous 
soil IS essential. When a potato starts its new growth and 
IS rapidly developing its new shoots and roots we find 
that the skin, a hard and impervious layer, becomes inter- 
rupted by numerous breathing pores which enable an active 
respiratory process to take place. These pores can easily 
be seen with the naked eye on the skin of a sprouting 
potato. They are equally clearly seen on the twigs of 
trees such as those of the horse-chestnut. On the leaves 
the pores through which the plant takes in various con- 
stituents from the air are not visible except with such 
miagnification as the microscope affords. 



8 



xAs the leaves of most plants are expanded m a drier 
medium than that which surrounds the roots, these pores 
naturally allow a large amount of moisture to escape par- 
ticularly in dry weather. If we cover a plant with a bell- 
jar we can see this moisture condensing on the sides of 
the glass, and in carefully conducted experiments we can 
actually measure the amount of water so lost. From such 
calculations we estimate that a fair-sized tree standing m 
the open may lose ten gallons of water in the course of 
a summer's day. This loss of water from the leaves might 
at first be thought to be an accidental phenomenon due 
to the possession by plants of delicate expanded leaf struc- 
tures. As a matter of fact, however, this process of trans- 
piration is of vital importance to the plant. In the 
first place it can be shown by experiment that leaf trans- 
piration exerts a considerable amount of suction, and it is 
in part by this means that vx^ater is raised in tall trees to 
the upper branches. In shrubs and herbaceous plants this 
suction alone would be sufficient to raise the water into the 
leaves and flowers, though the roots too are equally con- 
cerned in the ascent of sap. Nor must we consider the 
water which is given off by the leaves as so much waste. It 
is indeed important that some of the water absorbed by the 
roots should be driven off. For the water contained m the 
soil contains the mineral salts which are necessary for the 
plant, m very dilute solution, and these salts require there- 
fore to become concentrated in the plant. This can only 
be done by driving off some of the superfluous water and 
that is efi'ected by the process of transpiration. The leaves 
are therefore acting as a condensing apparatus and thus 
performing a very needful function. 

I trust that what I have said regarding the sensitive- 
ness of the various parts of the plant to external forces 
and surrounding conditions will have impressed the reader 
with the power of response inherent in plants, and this 
will indicate how by artificially selecting our medium and 
method of treatment we can considerably modify the 
course of development of the plants we cultivate. 



Chapter 2. 



ROOTS AND ROOT NUTRITION. 

Absorption by roots and root pressure. Physical and chemical 
nature of soil. Manures and their importance. Bacteria in 
soil. Root tubercles of Leguminous Plants, Rotation of crops. 
Trenching and ridging. 

We have seen that the sap is raised m plants in part 
by the transpiration of water vapour which takes place 
through minute pores which are found scattered over the 
surface, particularly the low^er surface, of leaves. In 
addition to this suction exerted by the leaves there exists 
<L definite upward pressure of the sap by the roots known 
as root pressure. This upward force is due to the fact that 
the roots of plants are covered in by a porous membrane, 
and that the cells of the root contain a cell sap of greater 
concentration than the water contained in the soil. When- 
ever such a condition of things occurs a -physical law 
determines that water passes through such a permeable 
membrane from the less dense to the. denser liauid. 
Water is thus attracted into the cells of the root and, 
causing them to swell, it forces the liquid up certain con- 
ducting channels of the root and into the stem. 

If, during a period of active root absorbtion, we cut 
a plant above the surface of the soil we find that water 
will be forced out of the exposed wound and, fixing a 
glass tube tightly in the place of the stem which has been 
removed, we find that the liquid which exudes can be 
forced up to a considerable height. 

Particularly in spring when root absorbtion is ver>' 
active the pressure exerted by the roots may be ^'ery con- 



9 



lO 



siderable and causes the bleeding " ot plants, as the 
active exudation of sap is called. This occurs sometimes 
when trees and shrubs are pruned too late in the spring 
after the sap has begun to rise. The active absorption of 
water takes place in the young roots a little way behind 
the root-tip, where the root can be seen to be covered bv 
a mass of delicate hairs which, possessing very thin walls, 
offer great facility for the penetration of the water. It is 
important, therefore, in transplanting young plants from 
a seed bed or pan to injure as little as possible the delicate 
young roots, as by pulling instead of carefully digging 
up young plants the absorbtive part of the root anchored 
by its many hairs may be left behind in the seed bed. 
Plants injured in this way will inevitably suffer a set- 
back, as they will not be able to absorb nutriment 
efficiently until they have developed new roots to take 
the place of those which have been injured. 

Let us now consider the nature of the soil in which 
the roots are growing. We have already seen that to be 
suitable for the growth of ordinary plants it must be well 
drained, whether we are dealing with a plot of ground 
or with pot plants. If the earth becomes water-logged 
the roots are deprived of the necessar}/ air and soon die 
away. The physical condition of the soil is therefore as 
important as its chemical composition.' In nature, soils 
are not all equally porous. Some therefore require the 
careful attention of the cultivator. Clay soils particularly 
are liable to retain too much water and need to be 
specially treated for successful cultivation. The retentive 
power of a soil depends largely on the size of the particles 
of which it is built up. Gravel or coarse sand allows the 
water to pass through it more rapidly than fine sand, and 
even the finest sand is composed of larger particles than 
those of clay. We can easily prove this by stirring up 
sand and clay in water. The particles of sand soon settle 
down and allow^ the water to clear while the water in 
which clay has been stirred up remains muddy for a long 
time. If, however, we add a little lime to this cloudy 
liquid we find that it clears rapidly. This is due to the 
" coagulation " of the very fine particles of clay which 
become collected into larger groups, and these being 
heavier than the single particles of clay, fall to the bottom 
of the water and allow it therefore to become clear. It is 
upon this action of lime upon clay that the practice of 



II 



liming heavy soils depends, as the admixture of lime 
causing the ultimate particles of the clay to become col- 
lected together, the ground becomes more coarsely grained 
and therefore more porous. It is then easier to work, being 
lighter and more open, and is also warmer. This physical 
change which is effected by lime is not the onlv benefit 
derived from its use. Near our large towns the soil is 
usually rendered acid by the fumes produced by the com- 
bustion of coal. In foggy weather this acidity of the 
atmosphere becomes very noticeable. It has been shown 
by experiment that such acid soil, and even ordinary soil^ 
when watered with acid rain-water collected in towns, is 
very detrimental to plant growth. This aciditA' can be 
removed by adding limxC to the soil, a practice therefore of 
the utmost importance to those who cultivate gardens or 
plots near our industrial towns. 

Heavy soils can also be lightened "b}' the admixture 
of sand to the soil. Good natural loams, which are easily 
worked soils, consist of fine sand with some clay and a 
little lime. 

Lastly, farviyard viannre^ quite apart from its im- 
portant chemical effect upon the soil in enriching it with 
many valuable food substances, is of great value in 
improving the texture of all soils. To sands it gives great 
water-retaining power, while it renders clay soils more 
porous and friable. 

Let us now examine some of the chemical needs of 
the roots of plants. By a chemical analysis of the ashes 
left after btirning plants it has been ascertained that they 
are mainly built up of fotir chemical elements, namely, 
hydrogen, oxygen, carbon, and nitrogen, to which must be 
added smaller quantities of lime (calcitim), mas^nesium, 
potash, sulphur, phosphorus, and iron. Of the four 
essential substances, the first two, hydrogen and oxygen, 
combined in the form of water are present in all soils in 
that form. Carbon, though present in the soil largelv in 
the form of lime or calcium carbonate, is mainly obtained 
by the plant through its leaves from the atmosphere, 
where it occurs as carbonic acid. Nitrogen, on the other 
hand, of which there is a large supply in the air, cannot 
be made use of by the plant in this free form, but is 
mainly absorbed in the combined form as nitrates by the 
roots. In comparison to the other substances dissolved 
in the water, the nitrates are the most important salts in 



12 



the soil as far as the nutrition of plants is concerned. We 
can realise therefore the usefulness of nitrate of soda as 
a fertilizer. In nature the nitrogen for plant food are 
largely obtained from the humus, or decaying vegetable 
matter, which is present in all soils. In cultivation, 
where plants are removed when they die down or where 
they are taken up to be used as vegetable, it becomes 
necessary to replace the humus, which would naturally 
form, by leaf mould or other decaying matter such as 
manure. The nitrogen contained in rotting manure or m 
humus is, however, not mainly in the form of nitrates. It 
IS contained in highly complex organic compounds, 
while ni manure it occurs largely in urea, a compound of 
ammonia. These organic compounds require to be acted 
upon by bacteria, ivhich are foitncL in the soil and in dung 
before they are available as plant food. On the other 
hand we find in the soil micro-organisms of another kind 
w^hich have the power of combining the free nitrogen of 
the air with the oxygen and ultimately build up the 
nitrates so important to plants. These nitrifying bacteria 
are constantly at work, and when a field lies fallow the 
soil will be found at the end of the fallowing to be richer 
in nitrogen compounds than it was at the beginning. In 
addition to these organisms referred to above, there are 
special forms which are al^^'ays associated with the roots 
of plants belonging to the pea family. If we pull up a 
pea or bean we find that the roots bear curious swellings 
called root tubercles^ which are found when examined to 
contain numerous minute bacteria. The roots must not 
be considered diseased, though they mav look like it. 
They are in a normal condition, and the bacteria inhabit- 
ing these nodules do not injure the plants but enrich 
them with nitrogenous material which they obtain from 
the air found in the interstices of the soil. It is therefore 
particularly important to keep the ground around ]>eas 
and beans open by hoeing, so that the roots may not only 
get the necessary oxygen for breathing purposes, but also 
the nitrogen they require for their nutrition. It is bv the 
activity of these nitrifying bacteria that leguminous 
plants, as those belonging to the pea family are called, can 
grow in very poor soil, that is to sav, in soil in which 
there are very few nitrogen compounds, and yet produce 
seeds which are very rich in nitrogen, and therefore very 
important as food for mankind. Of further interest m 



13 



connection with these plants is the fact that when they 
have yielded their crop they have not exhausted the soil 
o^ its nitrogen compounds, but will be found to have 
actually enriched it. This renders leguminous plants a 
valuable crop to alternate with other crops which deplete 
the soil of nitrogenous material. In agriculture it is often 
found expedient in the case of poor or exhausted fields to 
dig in a leguminous crop such as lupins or clover, which 
are often grown for the, purpose of enriching the soil. 

With a view to increasing the number of the nitrify- 
ing organisms in the soil attempts have been made in this 
and other countries to introduce more of these bacteria, 
particularly into pots or beds in which sweet peas or other 
members of that family are to be grown. In England 
preparations of these bacteria were distributed some years 
ago as " nitrobacterine,'' but the use of this preparation 
was not found profitable ni all cases, probablv as in many 
soils there is already a sufficient supply of these bacteria. 
Professor Bottomley has, however, now^ discovered a better 
way of cultivating these bacteria in peat, with which they 
can be easily distributed. Peat, which represents the 
partially decayed vegetable remains, differs from leaf 
mould or humus in undergoing decay under very w^et con- 
ditions. As a consequence it remains permeated with 
certain substances which render it acid and unsuited to the 
growth of most plants, though heaths, azaleas, rhododen- 
drons, and other members of the heather family grow 
well in peaty soil. When rendered alcaline, however, peat 
has been found to favour the development of roots, and 
therefore the whole growth of plants, and containing as it 
does a large amount of organic material, much of which 
is rendered soluble when alcaline, it has a high manurial 
value. This is said to be still further increased by inocu- 
lating the peat with nitrifying bacteria, which grow very 
vigorously in alcaline peat and thus increase the available 
plant foods. Bacterised peat," as it is called, is not yet 
on the market, and has therefore not been extensively 
tried, but experiments which have been conducted at Kew 
Gardens with pot plants, and on a farm near Norwich, are 
of considerable promiise.^ 

* ^' The Spirit of the Soil.^' An account of the nitrogen fixation 
in the soil by bacteria, and of the production of auximones as 
promoted by bacterised peat. By G. D. Knox. Constable. 
London, 1015. 



-4 

It should not be forgotten that there are numerous 
other micro-organisms in the soil, many of which are not 
only useless to plant life, but may be actually injurious 
particularly by preventing the full development of the 
nitrifying bacteria. Dr. E. J. Russell, of the Rothamstead 
Experimental Station, has discovered that if soil is par- 
tially sterilised, either by steam or by volatile disinfect- 
ants, some at any rate of the harmful organisms are 
destroyed, and the subsequent activity of the useful 
bacteria is greatly increased. The effect of such partial 
sterilisation has been found particularly beneficial in the 
case of richly cultivated soil, such as that in use in green- 
houses and market gardens, and sterilisation has been suc- 
cessfully adopted in many instances. The chemical 
antiseptic used by Dr. Russell was toluene, one of the 
coal tar products. Its action as well as that of steam is 
generally believed to destroy the numerous protozoa, which 
are microscopic animal organisms. Some of these are 
known to feed on bacteria, and are therefore very likelv 
destructive of the nitrifying bacteria.* 

Before leaving the subject of the nitrogen nutrition 
the chief function of the roots of plants, it should be 
mentioned that excessive use of nitrogenous manures has 
been found to render many plants more liable to disease, 
particularly such as are produced bv various fungi. It 
is well, therefore, to practice moderation. 

As regards other substances than nitrogen which it 
has been found useful to add to the soil as fertilizers, 
the most important are potash and phosphoric acid, the 
latter in the form of various phosphates, including bone 
meal. 

These plant foods must be used in differing propor- 
tions according to the richness of the soil, and also 
according to the requirements of particular crops. Speak- 
ing generally, gardens and allotments which receive an 
abundance of stable manure do not require much in the 
way of other fertilizers : but in heavy soils basic slag 
used occasionally will be found a useful w^ay of adding the 
necessary phosphorus, while nitrate of soda is often valu- 
able to push on early crops, such as lettuces, peas, etc. In 

See Reports on the Partial SteriHsation of Soil " Journal 
of the Board of Agriculture, January, 1912, January. igi3, and 
May, 1914. 



^5 

agriculture it is usual to practice a rotation of crops 
whereby plants with different requirements as regards 
mineral salts succeed each other, and with the periodic 
introduction of a leguminous crop prevent the exhaustion 
of the soil. Though this is not so necessary in horticul- 
tural practice where the soil is generally richer, yet it is 
equally useful here to vary the crops m different portions 
of the plot, firstly because as shown above peas and beans 
enrich the soil in nitrogen, and secondly because such 
alternation often puts a stop to certain plant diseases 
which can only propagate themselves when the same plant 
IS grown year after year on the same plot. This is the 
case with such diseases as " club root," of turnips and 
cabbages, and the "wart disease" of potatoes. 

Another feature of some importance must be touched 
upon. Probably every gardener has noticed that the 
deeper soil of his garden differs very considerably from 
the surface soil. On heavy land the subsoil, as it is 
called, consists partly of solid clay, and is not so porous 
and crumbly as the surface soil. It is also of less nutritive 
value for it contains little or no decaying organic matter. 
Both physically and chemically, therefore, it is less suited 
to the nutrition of plants than the surface soil. It is 
important, therefore, when cultivating deep-rooted plants 
to improve the subsoil. This is done by trenching, a 
process which may be regarded as the cultivation of the 
subsoil. The surface soil is lifted off by removing one 
spit of soil, and then the subsoil may be improved both 
chemically and physically by digging in farmyard 
manure, or other forms of decaying organic matter. In 
the case of clayey subsoil, the addition of lime should 
not be forgotten. If the clay of the subsoil is too solid 
it may be necessary to replace it by loam, or it may be 
improved by burning it, whereby it is rendered more 
friable. By thus improving the subsoil, a much better 
nutrition of deep-rooted crops will be secured. In the case 
of lighter soils, where there is less difference, at any rate 
physically, between the surface soil and the subsoil, it is 
a common and beneficial practice when trenching to put 
the surface soil at the bottom of the trench and cover it 
with the subsoil, to which manure has been added. 

Another useful process for the amelioration of soil is 
that of ridging, this is particularly beneficial in the case 
of heavy and tenacious soils. By digging up the soil and 



i6 

heaping it m parallel ridges a larger surface is exposed 
to the weathering, particularly to the action of frost 
during the winter. The effect of frost is to disintegrate 
the large masses of soil, and so to render the earth more 
permeable both to water and to air. By exposure to the 
atmosphere moreover the decay of organic matter in the 
soil is promoted, and thereby valuable nutritive matter 
is rendered soluble and available for plant nutrition. It 
is, therefore, sometimes recommended, before ridging, to 
spread any stable manure which is to be incorporated with 
the soil evenly over the plot. A trough is then dug two 
spades wide, one spadeful being deposited to the right 
and the other to the left of the trough. The soil of the 
latter can also be lifted, and turned, or merely forked over. 
This operation of ridging should be commenced in the 
autumn, and can of course only be undertaken on ground 
that IS to remain empty during the winter. 

Further information regarding the nature of soils and 
manures can be found in the following excellent books : — 

E. J. Russell. Lessons on Soil." Cambridge Nature 
Study Series, i/-. 

E. T. Russell. A Student - Book on Soils and Manures.'' 
Cambridge Farm Institute Series, igrs, 3/6. 

A. D. Hall. The Soil." John Murray, London, 5/-. 

A. D. Hall. Fertilizers and Manures." John Murray, 
London, 5/-. 



Chapter 3. 

STEMS AND LEAVES, 

Mechanical requirements of stems and branches. Twining and 
climbing plants. The food conducting function of stems The 
work of leaves and their structure. The wilting and recovery ot 
leaves. Protection of leaves against excessive drought. Direct 
effect of surroundings on leaf development. Hardening off 
plants. 

The stem is not as important an organ of plants as are 
the roots and leaves. The latter are essential for the 
nutrition of plants, and their particular work can only be 
carried on if the leaves are fully exposed to the sunlight. 
There is consequently considerable competition among 
plants for a place in the sun/' and in the course of 
evolution there has been a development of plants of 
increasing size, which by overlapping the smaller cues 
have been able to reach the light. This has led gradually 
to the production of trees and shrubs, but in herbaceous 
plants too, the stem and branches have the function of 
displaying the leaves to the best advantage. This and 
certain m_echanical principles which are involved account 
for the peculiarities of branching typical of different 
plants. In addition to the stem and branches the leaf 
stalks are also concerned in the ultimate positions in which 
the leaves are expanded. By varying growth in length 
and direction the leaves become so placed that there is 
little or no overlapping and the dovetailing of the various 
leaves causes the formation of a regular "leaf mosaic." 



17 



i8 



This is very clearly seen in the arrangement of the leaves 
on the horizontal branches of a tree, or m the case of ivy 
growing up a tree trunk or on a wall. Even m plants 
possessing- no stem, and in which therefore the leaves are 
found close to the ground forming a rosette, the leaves 
are found to overlap very little and particularly when rlie}' 
are stalked it can be clearly seen that yotuig leaves which 
are formed near the middle of the rosette do not overlap 
the older leaves, as the latter become pushed out more 
and more from the centre bv the elongation of the leaf- 
stalks. 

The function of the stem being to bear and display 
the leaves it follows that, to carry the foliage and to resist 
the strain which winds will exert upon leafy plants the 
stem has to meet certain ineclia'nical requireniejus. The 
rigidity, and at the same time the elasticity of the stem 
is attained by the development of certain thick-walied 
cells which collected into groups are so arranged as to 
give the greatest amount of strength with the smallest 
expenditure of material. These strengthening cells are 
displayed in plants on the principles which have been 
adopted by engineers m the mantif acttxre of rigid, and at 
the same time elastic structures. We have, fi.rstl\-, the 
hollow cylinder as is seen in grass haulms. The slender 
grasses can maintain their erect position by virttie of the 
mechanical properties of their stravs-. The bamboo cane 
is perhaps the most powerful of all m this form of con- 
struction. In other cases we lind the stems developing 
internally a system of girders which give them the strength 
they require. 

There are some plants in which the stem has not 
sufficient rigidity to grow erect as it does in most instances. 
In such cases the stem may trail along the ground, often 
covering a large area and the plant is then often modified 
as a shade plant unless it grows among very short vege- 
tion, as in the case of some mountain plants. Sometimes 
by using the support of the rigid stems of other plants 
twiners and climbers are able to reach the light though 
they have to compete with much stouter vegetation. 
Twiners, such as the scarlet runner and the hop, have no 
special climbing structures, but their slender stems ending 
in a heavy terminal bud are bent over to one side, and on 
close examination can be seen to rotate very slowh", either 
clockwise or counter clockwise. This circular movement 



19 

of the top of the stem causes it to grow round any upright 
or nearly vertical object m its neighbourhood, and then 
as the slender stem tries to straighten itself out it grips 
firmly the support round which it has grown. As the 
rotation is always m the same direction for a given species, 
it is necessary before giving such a plant any artificial 
aid to see in which way the rotation takes place, for if one 
twines the plant m the wrong direction it will unwind 
itself again when left alone. It is difficult to keep a 
twining plant to a horizontal course as it of itself will 
never twine round a horizontal support. Climbing plants 
on the other hand can fix their special climbing organs or 
tendrils to horizontal supports, partly owing to the sensi- 
tiveness of these organs to contact. In some cases, as for 
instance m the Virginia Creeper 'Amxpelcpsis) the contact 
stimulus causes the tips of the tendrils to swell up into 
sticky suckers, which enable this creeper to fix itself to 
a vertical wall. Tvlore frequently however when the ten- 
drils com^e into touch with some object, they grow round 
it by virtue of the fact that the side which is touched 
grows less rapidly, so that it curves round the support. 
After it has grasped the latter the tendril contracts spirally, 
and thus tightens the climber to its support. Tendrils are 
generally formed from parts of the leaf, the leaf -tips m 
peas, the leaf-stalks m the Xasturtiuin and Clematis.'" 

Besides carrying the leaves the stem has the further 
function of conducting to these leaves the food material 
absorbed by the roots. As we have seen the forces at 
Avork in connection with the supply of water to the leaves 
are, firstly, the root pressure lecture 2;, and secondly, the 
transpiration current in leaves lecture i^, which latter 
causes a considerable suction to be exerted on the water 
in the stem. The special channels through which the sap 
rises are the vessels, long continuous passages, running in 
the wood of die stem. But this upward passage of water 
with the inorganic salts it contains, is not the only con- 
duction required. The comiplex substances formed, as 
we shall see, in the leaves require to be conducted away 
partly to nourish the fxowers and fruits, partly to enable 
further growth and development of the roots to take place. 
This elaborated sap passes through other channels which 

^ Further information concerning this interesting group of 
plants can be obtained by the perusal of Charles Darmn's 
^' Movements and Habits of Climbing Plants."' 



20 



lie outside the wood and are spoken of as the bast-tubes. 
These are very dehcate, and to prevent them from being 
crushed they are protected on the outside by strong resist- 
ant fibres called the bast fibres or hard bast. 

With regard to the essential zvork oj leaves their 
primary function is without doubt to build up organic 
food material from the water taken up by the roots. This 
they are able to combine with carbon which they take 
from the atmosphere^ where it occurs plentifully in the 
form of carbonic acid, the gas we breathe out from our 
lungs. In the presence of sunlight all parts of the plant 
which contain the characteristic green colouring matter are 
able to break up the carbonic acid, retaining the carbon 
and giving back the oxygen which was previously com- 
bined with the carbon. In doing this they are constantly 
purifying as it were the air and enriching it with the 
life-sustaining ox}'gen needed by man and all forms of 
animal life. The carbon which the plants retain is com- 
bined With the elem_ents of water to form organic material 
called carbohydrates, of which starch and sugar are the 
most important to plants. 

Within certain limits the brighter the light the more 
active is this process of carbon nutrition, or assimilation 
as it is called. Hence the great importance of giving 
plants as bright and sunny a position as possible, and 
hence also the reason why plants seek to secure the 
most favourable positions for their leaves. The formation 
of leaf mosaics, mentioned above, is a good instance of 
means adopted to secure the most effective display of 
leaves. Unless specially adapted for growing in shade, 
as are for instance most ferns, the majority of plants will 
not show their best development tmless grown in an open 
situation. Apart from the essential influence of light upon 
the process of leaf nutrition, it has been found by experi- 
ment that this important ntitritive process is affected by 
temperature being within certain limits proportional to 
the rise in temperature. This explains why in sheltered 
places, where the air is heated up in winter many plants 
are known to make better growth than actually in the 
open. Shade can be partially counteracted bv w^armth. 
In frames in which plants often get less light than in the 
open, the warmer temperature enables them to grow more 
rapidly than were they exposed to the colder air. It 
must not be forgotten too, that not only is the process of 



21 



leaf nutrition stimulated by warmth but root-absorption 
also increases with a rise in temperature, so that all nutri- 
tive processes are favoured by warmer conditions of soil 
or climate. 

A secondary but important function of leaves was re- 
ferred to in a preceding lecture, namely, the importa^nce 
of getting- rid of a certain amount of w^ater vapour mto 
the atmosphere. This process of transpiration aids the 
upward passage of sap and also causes the concentration 
of the necessary mineral salts, which are only contained 
in dilute solution in the soil. 

Let us now consider how the structure of leaves is 
adapted to perform its important functions. For both of 
them it is essential that the air should be in close contact 
with the cells of the leaf. It is essential to prevent too 
great a loss of water from the leaves and also for pro- 
tective purposes it is necessary that the surface of the 
leaf should be covered with a more or less resistant layer. 
Consequently the passage of air to the interior of the leaf 
is effected through microscopic pores called stomata, which 
are usually found in greatest number on the under sur- 
face of leaves. These pores have the power of opening 
and closing so that the aperture can be regulated to the 
needs of the plant. They open with the approach of day- 
light and thus enable the carbonic acid to enter the leaves 
when the conditions are favourable for leaf nutrition. At 
the same time, of course, water vapour escapes outwards 
through the pores. The inner parts of the leaf are so 
arranged that the cells containing most of the green colour- 
ing matter are near the upper surface of the leaf where 
they will receive most light . Throughout the interior of 
the leaf there are vvide spaces through which the air can 
circulate readily between the cells. The veins w^hich bring 
up the supply of water and which also have to collect 
and conduct away through their bast portion the organic 
material formed by the leaf, become very finely branched 
so as to be in contact with all parts of the leaf. 

If w^e consider that the larger the leaf surface the 
larger the amount of light absorbed and hence the greater 
the nutrition, we might wonder why all plants have not 
larger leaves. The limitation in the size of leaves is 
largel}/ due to mechanical considerations. A slender stem 
cannot bear leaves beyond a certain size. Aloreover, 
though a large leaf would increase the process of carbon 



assimilation, the loss of water from a large leaf surface 
mig-ht be more rapid than the absorption of water by the 
roots. Whenever this happens it causes the ivilttng or 
drooping of leaves, and should that occur too frequently 
the plant will not survive. As it is, wilting of leaves m 
the open occurs m hot summer days and affects mainly 
plants with large delicate leaves, large at all events in 
proportion to their roots, such as Doronicums, Calceolarias, 
and young Cauliflowers. In the case of pot plants, which 
are not only losing water through their leaves but also 
through the sides of the porous pots, it is a common 
phmomenon to find plants wilting during the heat of the 
day, and to prevent this it is necessary to shade the houses 
during the summer months, at least on very bright days. 
Wilting IS particularly noticeable in recently transplanted 
plants, in which there is always sure to have been some 
injury to tlie roots so that absorption cannot keep pace with 
transpiration. Transplanting is therefore best undertaken 
in wet or dull weather, or at the end of the day, when 
during the succeeding night the stomata of the leaves v\-ill 
be closed, and conseo;aently at the start, at all events, there 
will be no undue loss of water. Cuttings which have not 
yet produced an adequate supply of roots require in 
many cases to be kept in a moisture-laden atmosphere in 
a frame. It is interesting to note that when plants in the 
open flag at noon on a hot summ.er^s day, they recover 
again at nighl even without further water supply, \\-hen 
the leaves droop the stomata usually tend to close, and 
at night they are certainly closed so that the roots have 
then the cliance of replenishing the drooping leaves with 
water. V\'atering the plants while the sun is still on them 
should be avoided. Drops of water on the leaves are apt 
to act as lenses and focussing the rays of lic^ht upon the 
leaf often cause burning of the tissues and consequent 
spotting of the leaves. Watering the soil has also some 
dangers unless properly done. ^Merely dam.pmg the sur- 
face of the soil, apart from the tendency to cause it to 
cake, stimulates the growth of surface roots, which vrill 
constantly require watering in dry weather. It is better 
to give plants a good soaking from time to time. The 
water penetrating to some depth causes the development 
of deep roots, which will keep the plant supplied with 
water even m dry weather. 

It is often thought that leaves, or at an}' rate the leaves 



23 



of some plants, have the power of absorbing- water. It 
is not an uncommon practice to spray the leaves of plants 
in the greenhouse when they are shaded, and also in the 
open when the sun is no longer upon them and their sub- 
sequent recovery, if they were drooping before, is attri- 
buted to their having absorbed water. This is however 
not the case. The spraying of the leaves, besides cooling 
the foliage, which is often considerably heated by the sun, 
has the effect by the evaporation of the water to render 
the atmosphere around the plant moisture-laden, where- 
fore the transpiration of the leaves is at once decreased. 
It is this decrease of transpiration and not absorption by 
the leaves which causes the revival of the plants. No 
leaves with an impervious covering can take m water and 
there are very few leaves which are not so protected. 
Mosses and Filmy Ferns alone among leafy plants can 
take in water through their leaves, which are adapted to 
growth in moist if not dripping conditions. 

It is interesting to note that when the stomata are 
closed and transpiration is impossible, some plants are still 
able to get rid of their superfluous water. At night the 
leaves of the Nasturtium and the Fuchsia for instance are 
able to force out little drops of water through water-pores, 
which are found at the termination of their veins. Many 
grasses, too, liave water-pores at the tips of their leaves, 
and the drops of water exuded by them often look like the 
formation of dew. 

When growing m very dry soil or in hot arid climes, 
plants may experience a great difficulty in supplying their 
leaves with water, often during a \-ery prolonged period. 
In such cases many remarkable deviations from the normal 
structure are prod'uced, with the object of frotecting the 
leaves against drought. In most cases the leaves are small, 
often reduced to micre scales or needle-shaped structures. 
Such are the leaves of man}' of our moorland plants grow- 
ing m sandy, wcii-clrained soil and exposed to strong and 
drying winds. In still more arid regions the leaves may 
disappear altogether, and we get such curious plants pro- 
duced as the fleshy Cacti of the New World and the 
succulent Euphorbias ^Spurges) of the Old AA'orld. In 
both of these groups of plants the green stems undertake 
the process of assimilation, and they also store up sufficient 
water during the wet season to enable the plant to last 
out during a prolonged drought. 



24 

It is sometimes stated that these strange modifications 
are the result of the direct action of the environmxent on 
vegetation. This is impossible of proof, and we may 
assume that it is largely by natural selection of forms or 
varieties most suited to these extremes of climate that 
desert plants have m the course of ages sprung from 
ordinary types of vegetation. Nevertheless we must not 
forget that to some extent climate has a direct modifying 
action. If, for example, we grow a seedling gorse in a 
moist greenhouse, we find that it will persist for a long 
time- in producing small ordinary leaves, which precede 
the spines m the young plant, and this effect is certainly 
a direct action of the surrounding conditions. It is also 
well known that the texture of leaves produced by plants 
growm in a greenhouse differs greatly "from that of leaves 
of the same species growing out of doors. It is necessary 
therefore before transplanting into the open plants raised 
under glass, to get them gradually acclimatised to their 
new surroundings. By transferring them from greenhouse 
to frame and keeping the latter well ventilated, or by 
placing plants in a sheltered place m the open, the leaves 
have an opportunity of becoming Jiardeiied to more 
rigorous conditions of existence. the outer layer of the 
young leaves can become strengthened, and they as well 
as the new leaves will all be modified to suit the new 
environment. In transferring a plant from a dry to a very 
moist house it will often be noted that the older leaves 
are shed. Being fully developed they are unable to adapt 
themselves to the new conditions. Not able to transpire 
freely owing to the moisture-laden atmosphere they become 
overcharged with water and this probably causes them to 
fall. The new leaves will be suitably m.odified to suit 
the more humid conditions. 



Chapter 4. 



METHODS OF VEGETATIVE REPRODUCTION AND 
PROPAGATION. 

Tubers. Bulbs and corms. Bulbils. Runners. Layerings and 
cuttings. Budding and grafting. 



As explained in my first lecture vegetative refro- 
diiction is the multiplication of the plant by mxeans of 
structures, which partake of the nature of vegetative organs 
and are not the result of the fertilisation of flowers. 
This means therefore that the offspring, however numerous 
they become m the course of years, are not so much 
descendants as actual portions of the original individual 
just as the most recently formed buds of a tree are really 
part of the same tree, which ten or a hundred or even a 
thousand years before bore similar buds. The only 
difference is that the buds which the tree produces }'ear 
after year all remain attached to the same parental stem, 
while m the case of tubers or bulbs the parent plant dies 
dow^n each year, so that these structures becomic so m.any 
separate individuals. The very large number of distinct 
plants which thus arise will perhaps best come home to us 
if we consider that in the case of a potato plant pro- 
ducing yearly only six new tubers, we should have at the 
end of ten seasons obtained over ten millions of tubers. 
If an average of ten tubers were produced by each plant, 
the number of tubers at the end of ten generations would 
be 10,000,000,000 potatoes. As the millions of new plants 
which are thus developed are, properly speaking, all parts 



25 



20 



of the same plant they should, apart from slight differences 
of nutrition, not only be all similar but actually of the 
same constitution. It is true that bud variation does 
occasionally arise in plants, but it is a comparatively rare 
phenomenon, and consequently vre find that as a rule 
plants, raised by vegetative methods, maintain the char- 
acter of the parents and do not show the sporting with 
which one is familiar when raising plants from seed. The 
purity of the strain is thus easily maintained by vegetative 
propagation. 

Let us now examine some of nature's methods of 
vegetative reproduction. In the hrst place, we have such 
well-known examples as tubers which form the chief 
method of propagation of the potato and of the Jerusalem 
artichoke. A tuber is the swollen-up end of an under- 
ground branch. This can be clearly seen by digging up 
a young potato plant and following these subterranean 
shoots from the parental stem to their tuberous tip. Both 
the branch and also the tuber show the rudiments of 
leaves, reduced to small scales and spirally arranged 
round the potato. On an old potato the sickle-shaoed 
mark below each eye or lateral bud reoresents the insertion 
of the leaf or leaf scar, and at one end of the potato where 
these beccnie more crowded together, we have the terminal 
bud of the tuber. This is called the ''rose" end. At the 
opposite end we can generally find the scar or heel where 
the branch of which the tuber is the dilated tip, was joined 
to it. In all cases tubers are filled with stored food 
miaterial, mainly starch in the case of the potato. AVhen 
potatoes are set m the spring, the terminal bud grows 
up to form the main stem of the new plant, while the 
other eyes remain generally dormant. The new tubers are 
formed from specialised underground branches produced 
from the lower region of the mam stem. The latter is 
therefore often earthed up so as to prom.ote the develop- 
ment of these side shoots. It is a very common practice 
before setting potatoes, to place them side by side with 
the " rose '' end upperm_ost and allow the term.inal shoot 
to commence its development in dayli,a-ht. By this means 
the shoot does not elongate so rapidlv as when growm 
underground, that is, in the dark, and thus the lateral 
buds, which will develop into tuber producing shoots, are 
miore closely crowded together and more numerous. This 
method therefore results in a greater yield of new tubers. 



As there is generally a superabundance of food 
material, it is possible to divide a tuber into several pieces, 
and provided each piece has enough food material and a 
sound " eye," it will produce a new plant. It has been 
found by practice that when cut for " sets " the pieces near 
the rose " end give the best results. When a potato is 
divided up in this way it is necessary to leave the separate 
pieces spread out to dry before they are planted, so that 
they may form a protective layer before coming in con- 
tact with the soil, the bacteria of which might cause them 
to decay. 

• Recently a method of raising new potatoes in the dark 
has received prominence in the press, and in many quarters 
erroneous views have been formed concerning this mode 
of cultivation. It has been thought by some that potato 
plants could grow like mushrooms and were not dependent 
upon light for their full development. This is very far 
from being the truth. Of course it is well know^n that 
potato tubers will sprout m the dark, as they do indeed 
in nature underground, but they are only able to grow 
without light while there is still a supply of food material 
in the old tuber upon which they can draw^ for their 
development. When that is exhausted the plants must 
inevitably die, as w^ithout light they are like all green 
plants unable to manufacture new organic material. It 
has however been found by experiment that if tubers are 
then half buried in fine dry soil spread out on a table 
in the cellar, they will soon be surrounded by a crop of 
small new potatoes close up to the old tubers. Large 
tubers should be used for this method of cultivation, and 
they should be placed three or four inches apart. It must 
of course be remembered that these small new potatoes 
are produced entirely at the expense of the food material 
stored in the old potato. We are therefore by this method 
of cultivation not increasing the food supply of the 
country but merely replacing old potatoes by a crop of 
more palatable new ones. 

Bulbs and conns, w^hich are the vegetative methods of 
reproduction of many members of the Lily and Iris 
Fam.ilies, must be looked upon as specialised underground 
buds, which by virtue of their store of food material are 
able to lead an existence independent of the plants on 
which they have been produced. Like the ordinary winter 
buds of a tree, such as the horse-chestnut, they are pro- 



28 



tected on the outside by a few dry scaly leaves, while at 
their centre will be found the foliage leaves and flowers 
of the next season. But between these we find another 
set of leaves w^hich do not occur in the buds of a tree, 
namely, thick storage leaves of a fleshy nature. It is the 
possession of this internal supply of food material which 
enables these specialised bulbs to produce their leaves and 
flowers in the next season, though separated from the 
parent plant which formed these bulbs, while in the case 
of the tree the winter buds expand in spring by making 
use of the food material stored in the branch to which they 
belong. 

What is the nature of the parent plant on which the 
bulbs are borne as lateral buds ? Take up a tulip plant 
after it has flowered and you will find at the base of the 
upright stem bearing the leaves the storage scales whicli 
have made the growth of the stem and leaves possible, 
and at the base of one or more of these scales will be seen 
small buds, w^hich are beginning to sw^ell owning to the 
organic material manufactured by the leaves, passing 
down to them.. This observation will teach us that it is 
important, if we wish to save our own bulbs for planting, 
to leave the old bulbs in the ground for some time after 
flowering and not to cut off the foliage leaves but to pre- 
serve them as long as they are fresh and green and capable 
of manufacturing food material. This is the time, too, to 
give the plant further food in the form of top dressing 
or occasional supplies of manure water. It is also im- 
portant to cut away the dead flowers, so that the food 
material is not expended in the ripening of the seed vessel. 

Another point of im.portance in bulb growing is to 
secure the proper ripening of the bulb. In nature bulbs 
after losing their leaves pass through a resting stage, 
during which they are kept dry by the vegetation cover- 
ing the soil in which they grow^, whether they occur m 
v/oodlands or m.eadows. In our gardens where the ground 
above themx is usually uncovered, water percolates down 
to them, and if they are not very deeply buried slugs, too, 
may attack them. It is therefore often advisable in the 
case of damp soils, to take up the bulbs when the leaves 
are dying away, and to allow the bulbs to dry in the sun, 
storing them afterwards in a dry place until the time for 
planting arrives. 

Cornis are \ ery like bulbs and may often be mistaken 



29 



for the latter, but on cutting them across we find that the 
dry scales which surround them on the outside are not 
followed by fleshy storage scales, all the food material 
being stored in the thick solid stem which makes up the 
bulk of the corm. In their growth and development, 
however, they follow^ very closely the life history of the 
bulb. Crocuses and Gladioli possess corms, while Lilies, 
Tulips, Hyacinths and Daffodils have true bulbs. The 
bud-like nature of bulbs can be clearly recognised by 
examining the small sv^'ollen buds or bulbils which arise 
on the aerial stems of certain species of Lily, and which, 
though unable to give rise immediately to a new flower- 
ing shoot, can gradually be grown on to produce in time 
a mature bulb. 

R?/amers, such as are produced in the Strawberry and 
Violet, are delicate lateral shoots creeping over the surface 
of the soil and becoming readily rooted at their nodes 
when in contact with the moist soil. In nature they cause 
the very rapid spreading of these plants, and by the decay 
of the portion of the stem w^iich joins them to the parent 
plant they may cause the increase of individuals. They 
are conveniently used for propagating purposes and 
should always be removed from the parental stock as they 
draw nutriment from it and therefore impoverish the latter, 
with the result that they reduce the number of flowers. 

Many plants which do not possess natural means of 
vegetative reproduction can be caused to give rise to new^ 
individuals by separating certain portions, generally 
lateral shoots, and inducing the same to develop new roots. 
In some instances the formation of these roots is promoted 
before the branch is separated from the plant ; this process 
is known as layering. The lower branches of such shrubs 
as Gooseberries and Red Currants may be bent down and 
partially embedded in a shallow trench dug round the 
bush, and filled with light and porous soil. When stimu- 
lated by moisture the buried portions of these branches, 
the tips of which must be allowed to project beyond the 
trench, wdll produce what are termed adventitious roots, 
and when these are sufiiciently well-established, the branch 
may be severed from the parent plant, and the new 
individual w^ll lead an independent existence. It is 
generally found advisable to cut back the projecting por- 
tion of the branch to two or three buds. This method 
can also be em.ployed for Rhododendrons and for more 



delicate shrubs, as well as for climbers like Clematis. A 
similar process has been found advantageous for miany 
herbaceous plants such as Violas, Carnation and Pinks. 
In these cases, however, it is usual to make a slight 
incision in the buried portion of the shoot, which should 
always extend to one of the knots or joints. Such an 
incision stimulates the production of roots, particularly at 
the end furthest removed from the parent plant. This is 
no doubt due to the accumulation at this point of food 
material which has been produced by the leaves at the tip 
of the layered shoot, the food material being unable to 
be carried across the incision. Even in dealing vs'ith 
shrubs it is sometimes found advisable to make an in- 
cision partially across the branch which is to be layered, 
and to peg it down so as to keep it in position until 
the new roots are formed. It is of great importance that 
the soil in which the new roots are to form should be well 
aerated, more particularly in the case of herbaceous plants 
which are more liable to injury by rotting. It is advis- 
able, therefore, to add a considerable amount of sand to 
soil in which layerings are to be embedded. 

In the case of upright stems or branches, which are far 
above the soil, the production of adventitious roots can be 
promoted by removing a ring of outer tissues to the depth 
of the wood, and then tying a handful of wet moss round 
the shoot. 5uch ringing causes the food material formed 
111 the upper part of the plant to accumulate above the 
wound, and this promotes the rapid development of roots. 
When these have become sufficiently established the 
rooted portion of the stem or branch may be severed from 
the parent. 

In many plants the formation of adventitious roots is 
so rapid that shoots completely detached from a plant 
can establish themselves as cuttings, producing their own 
roots when placed in suitable conditions. In all cases in 
which cuttings are made the first need is to cause the cover- 
ing in, that is, the healing of the cut end of the shoot. 
This is done by the development from the actively grow- 
ing cells of a peculiar wound tissue, called callus, which 
by growing over the wound and developing a layer of 
cork, prevents the destruction of the exposed cells and 
the entrance of harmful bacteria. The growth of callus 
is promoted by the aeration of the tissues, and it is there- 
fore important that the soil in which cuttings are placed 



31 



should be even better drained than that supplied to normal 
plants. A soil made porous by the plentiful admixture 
of sand should always be selected, and m the case of 
cuttings which are being struck in pots, it is a common 
practice to insert them close against the inside of the 
porous pot, through w^hich the air has a free access to the 
cuttings. The danger of over w^atering cuttings is even 
greater than in the case of well-established plants. Cut- 
tings taken from woody plants produce their wound 
tissues as well as the adventitious roots, w^hich are formed 
later, at the expense of the food material which is stored 
in the shoot. They are not therefore so dependent on 
ivarmth and light in the early stages as are herbaceous 
cuttings. They can indeed be struck in the autumn or 
w^inter after the leaves have fallen, or in the early spring 
before the foliage has been developed. If taken in the 
autumn they often do not produce their roots until the 
following spring, and they are alw^ays later in the develop- 
ment of their leaves than are well-established plants. In 
the case of herbaceous cuttings which have no store of 
food material, it is necessary that they should be able to 
continue to form new organic food material in their leaves 
so as to promote the growth of callus and the development 
of roots. It is esential therefore that they should have 
plenty of light ; but in the first few days before they have 
adapted themselves to their new conditions, they are liable 
to lose too much water by evaporation, and it is important 
during this period to keep them slightly shaded, or to 
grow them in a moisture-laden atmosphere in a closed 
frame or greenhouse. It is better to prevent the loss of 
too much w^ater by protecting the leaves in this way than 
by excessive supply of water to the soil, as herbaceous 
plants are very liable to decay by the action of bacteria 
on the cut end of the shoot. As herbaceous cuttings have 
to continue to manufacture food material, they also require 
a greater amount of heat than do woody cuttings, the 
process of leaf nutrition being stimulated by an increase 
of temperature. Herbaceous cuttings must, therefore, not 
be taken too late in the autumn unless they are to be grown 
in artificial heat. 

A few plants can be raised from root cuttings. This 
is possible where plants are endowed by nature with the 
power of forming adventitious buds on their roots. Rasp- 
berries, Pears and Apples are all exam.ples of plants which 



32 



often produce suckers from their roots in the neighbour- 
hood of the parent plant. Such growths can be separated 
and developed into new individuals. In the case of Rasp- 
berries indeed, this is a common method of propagating 
the canes. 

Lastly, it has been found possible in the case of some 
plants vvith somewhat fleshy leaves to cause thesie, or even 
portions of a leaf, to produce adventitious buds. This is 
the case with many Begonias, particularly those belonging 
to large-leaved varieties. If the leaf is placed on damp 
soil, the midrib having been cut in several places, new 
plants may arise from each portion as with the stimulus 
of warm temperature and moisture the leaf produces a 
considerable growth of callus, from which adventitious 
buds soon arise. The fleshy scales forming the bulbs of 
most lilies are capable when separated from the parent 
bulb of producing small adventitious buds from which 
new plants can be grown, and this is a common method of 
propagating the plants. 

Space prevents a detailed discussion of the processes of 
bitddiJig and grafting, but from the botanical point of 
view the processes may be regarded as a special case of 
making cuttings in which the latter, instead of being 
planted in soil, are inserted in the tissues of a nearly 
related plant with which they become united by the 
development of wound tissue or callus. No adventitious 
roots are formed by the graft as the scion relies for its 
supply of water entirely upon the roots of the stock. 

There has been much discussion as to whether, as a 
result of grafting, there is any influence of the stock upon 
the scion, or vice versa. A considerable amount of infor- 
mation on this question has accumulated, but it is largely 
of a negative character, and what positive evidence exists 
is of a doubtful nature. 



Chapter 5. 



FLOWERS AND THEIR FORMATION. 

Conditions favouring the production of flowers. Structure and 
functions of the various parts of a flower. Pollination and 
Fertilisation. Self -fertile and self-sterile flowers. Ripening of 
fruits and seeds. 

In speaking- of the difference between vegetative and 
reproductive organs, in the first chapter I have already 
mentioned that certain external conditions which favour 
the development of the former affect adversely the forma- 
tion of flowers. Long horticultural practice has also 
proved that the developineni of floral organs can be 
stimulated in various ways which are well known to 
gardeners. Thus reduction m the supply of water and 
consequent stoppage of the luxuriant development of 
leaves is one of the chief methods employed. It has been 
found by scientific experiments that these various horti- 
cultural practices are based upon definite physiological 
requiremients. In recent years much has been done to 
confirm and extend our knowledge of this subject. It 
was proved that bright illumination of plants is essential 
for the production of flowers. At first it was thought 
that certain rays of light influenced the development in 
the plant of special flower-forming substances, and early 
experiments seemed to indicate that these were produced 
by definite rays of light from beyond the blue end of the 
spectrum — rays of light w^hich are known to have great 
chemical activity. More recently, however, it seems to 



33 



34 



have been established that it is probably the intensity 
rather than the quality of light which is required for 
flower production. Another factor which is important m 
this respect is the co7icent7ation of organic material in the 
stem and leaves of the flaunt ^ while an increase of water 
and inorganic salts tends to the development of foliage 
rather than flowers. It is for this reason that a reduction 
in the supply of water is so helpful in producing abundant 
flower buds. It is som.etimes thought that a wealth of 
blossom on our fruit trees predicts a fine summer, from 
what has been said above^ it is obvious, however, that it is 
the fine summer or dry autumn of the previous year which 
is responsible for the prolific bloom. Flower buds on our 
fruit trees are, of course, already developed in the late 
autumn. Dealing with herbaceous plants, it has been 
found possible to arrest the development of flowers even 
if the flowering shoot has commenced to make its appear- 
ance. Thus in specimens of the common House Leek a 
rosette of vegetative leaves may be caused to appear on 
the flower shoot if the plant is copiously watered and 
illuminated w4th light passing through a red or blue 
screen. Conversely, the runners of some plants may be 
made to bend upwards and develop flowers if water is 
w^ithheld and the plant placed in a very bright light. 

The horticultural practice of pruning and root pruning 
is also intended to further floral development. Each tree 
or shrub must be treated differently according as to 
whether flowers normally make their appearance on long 
shoots or on spurs, i.e., on new or old wood. The removal 
of non-flowering shoots is therefore what is aimed at. In 
summ.er pruning the stoppage of the growth of purely 
vegetative shoots Vvdll actually stimulate the older 
branches to form flowers, as they will have aji increased 
amount of food material at their disposal. The increased 
production of flowers effected by root pruning is often 
remarkable. It will be sometimes observed that apple 
and other fruit trees produce long and vigorous new 
shoots, particularly in an upward direction, which are 
caused by the development of deep roots able to obtain 
an abundant supply of water even in fairly dry soil. The 
removal of these deep roots stimulates the growth of 
fibrous roots in shallow soil, where there is a less abundant 
supply of water in the summer and early autumn; the 
result of root pruning is therefore the production of 



35 



numerous spur shoots, on which alone flowers are produced 
in the case of the apple and pear. In some of our orna- 
mental shrubs, however, such as Guelder Rose, Weigelia 
and Forsythia, flowers- are borne on the shoots of the pre- 
vious year, and it is consequently a mistake to cut these 
back in the autumn, as is so often done thereby reducing 
the beaut;/ of these shrubs. Old wood, on the other hand, 
and branches which have already flov/ered, should be 
pruned av/ay at the end of the summer. Some plants 
which are nearly related, differ in the m.anner in which 
they bear their flowers and fruit, and it is therefore very 
important before pruning to know^ exactly on which kind 
of branches the flowers will be borne. For instance, the 
Morello Cherry bears its flowers all along the shoots pro- 
duced during the previous sumxmer, while others flov/er 
at the base of the shoots on short spurs. These kinds 
must therefore be pruned differently from the former. A 
similar difference is found in the case of currants. Black 
Currants flower all along the shoots formed in the pre- 
vious summ.er, while Red Currants produce their fruits on 
small spur shoots found on older wood. 

We must distinguish between the methods causing the 
production of flovver buds as described above, and prac- 
tice — such as forcing, which has for its object to cause an 
early unfolding of the same. Unless bulbs or aerial buds 
already contain the rudiments of flow^ers no amount of 
forcing will cause these to be formed. The application 
of moist heat, often m faintly lighted pits, is indeed 
inimical to the production of flower-forming substances 
and promotes vegetative growlh rather than flower pro- 
duction. On the other hand, if flower buds are present 
their expansion can be materially accelerated. By these 
means flowers can be obtained in mid-v\anter when they 
will be particularly appreciated. }Jost storage organs, 
whether tubers, bulbs, or winter buds, require to pass 
through a resting period before they enter upon a new 
period of growth. Various methods can be adopted there- 
fore to accelerate either the process of ripening the buds 
or of reawakening the dormant structures. While dr3mess 
promotes the former, heat and moisture effect the latter. 
It has been found however that the resting period can be 
shortened by various mieans. For example particular 
varieties of potato, which will not sprout in the autumn 
and cannot therefore be used for the culture of earlv 



36 



p>otatoes, if placed for a fortnight in a cold chamber 
slightly above freezing point will readily develop when 
planted. A similar treatment is advantageous for 
rhubarb plants which are to be forced. Lift them out of 
the ground and let them dr}' and be exposed to light 
autumn frosts and they will then respond more readily 
to methods of forcing. This practice of thoroughly cool- 
ing plants which are to be forced is now regularly used 
m the case of bulbs, retarded Lily of the Valley crowns, 
Lilacs, Spireeas, etc. Abroad, other methods have come 
into vogue with the same object m view. It has been 
found that the mimersion of the branches of plants for ten 
to twelve hours in a hot-water bath of from 85 deg. to 
100 deg. Farenheit, has a remarkable etrect in accelerating 
the unfolding of winter buds. The roots should not be 
so treated. It is therefore best to invert the plants and 
allow the stem and branches to hang down into the hot 
water. The effect of the latter is quite local ; so that 
when partially immersed only those buds which have been 
under water are affected. Six weeks after this treatment 
lilacs will be 111 full bloom, if subsequently grown in the 
usual wa}'. Professor j ohannsen, of Copenhagen, has dis- 
covered that exposure of plants to ether vapour for twenty- 
four to forty-eight hours has a similar accelerating effect. 
Both methods are largely used on the continent. 

Let us now consider the struct (ire of Hoivers and the 
fitnctioji of their various farts. So manifold is the 
appearance of flowers that it might seem at first difficult 
in a short space to make any general statement on their 
structure ; yet as regards the essential organs of reproduc- 
tion we find considerable agreement. It is more par- 
ticularly in the structure of the brightly coloured petals 
that w^e find great variety, and largely owing to the 
adaptation of the flowers to the visits of insects. The 
colour and scent is developed to direct them to the hone\' 
which the flowers provide, and special honey guides, spots 
in the Rhododendrons and lines 111 Tansies and Violas, 
as well as difl'erences in colour, guide them to the nectaries. 
In making their way to these the insects come in contact 
vnth the delicate stamens of the fiower and the pollen 
contained m the pollen sacs at the tip of the stamen 
becomes dusted on to the insect. Vlien it visits the next 
flower, some of this pollen will be dusted off on the stigma 
at the top of the immature seed vessel, and thus the insect 



37 



effects the pollination, which is the necessary preliminary 

to the feriilisation of the flower. The pollen grains left 
on the sticky surface of ihe stigma germinate there, and 
a delicate tube grows clown from them to the immature 
seeds, which become fertilised by the fusion of the vital 
element of the pollen gram with that of the ovule. Even 
though a flower may produce both of these essential 
organs, the pollen producing stamens and the ovule con- 
taining seed vessel, self-pollination does not usually take 
place as the stamens and seed vessel mature at different 
rimes. This can easily be seen in such a large flower as 
that of the Nasturtium, m v-hich the stamens may be 
observed ripening one after the other and liberating their 
pollen, and only after the last of the pollen sacs has shed 
its pollen doe- the three-pronged stigma open and is then 
ready to receive pollen brought from another flower m 
which the stamens are opening. All the various and 
wonderful mechanisms, by means of which flowers ensure 
their pollination, have the purpose of securing the ferti- 
lisation of the ovules with pollen from another flower and 
if possible from another plant, for as Darwin has shown * 
cross fertilisation usually causes the production of more 
numerous and stronger offspring than self-fertilisation. 

Gardeners v\'ho grow exotic plants, the flowers of which 
are adapted to the visits of insects not found m this 
country, have often to perform this service themselves and 
to pollinate artificialh' the flowers using usually a flne 
camel-hair brush. In plants like the Tomato, tapping the 
stems with a stick, wrapped round with a cloth to prevent 
injury to the plant, will generally cause the pollen to drop 
out of the_opened anthers on to the stigma of these pendant 
fl.owers. If tomatoes are grown under glass, we must take 
care that during the period of flowering the house is kept 
dry, as the pollen sacs will not open m a moist atmosphere 
and the plants cannot be pollinated. Of course by the 
method described self-pollination only can be effected, 
crossing can be obtained by using a flne paint brush and 
pushing the hairs up between the stamens fl.rst of the 
flower and then of another. 

In some plants, such as the Cucumber and ^larrow, the 
flowers are of two kinds, som.e producing the seed vessel, 

Darwin, C. The Effects of Cross and Self-f'ertilization in the 
Vegetable Kingdom.'' (Murray, g/-.) 



38 



which can be seen as a thick structure below the petals, 
and others producnig- only pollen. In such cases, self- 
pollination IS hnpossible and cross pollination must be 
effected either by insects or by the gardener. It is curious 
that m spite of the many adaptations to cross fertilisation 
there are many plants m which self -fertilisation is the 
normal process of seed production. This is the case with 
most of our cultivated grasses. In the wheat and other 
cereals, though the flowers open and the light powdery 
pollen IS carried from plant to plant, the stigma of the 
seed vessel is usually pollinated before the flowers open, 
and as there is only one seed in each seed vessel it is 
usually self-fertilised before the foreign pollen arrives. 
Even m fiowers, apparently specially adapted to the visits 
of insects such as the Sweet Pea, which possesses both 
scent and colour, the immature pod is alreadv pollinated 
in the bud stage of the flower, and if we wish to effect 
any crossings between different varieties of this plant, we 
must cut away the stamens before the flowers open and 
introduce pollen from another plant. Wliile such normal 
self-pollination has for horticulturists the advantage that 
it preserves the purity of the varieties we cultivate, it is 
probable that it may gradually cause a weakening of the 
race, and in nature even occasional crossing probably re- 
invigorates the strain. 

Quite a number of members of the Pea Family are 
self -fertile, including the commonly cultivated forms of 
peas and beans, while many other leguminous plants, such 
as the clovers are self-jertile and cannot be fertilised with 
their own pollen even when it is placed on the stigma of 
the flower. An interesting analysis has been made of the 
behaviour in this respect of the various members of this 
Family, and it has been found that all the annuals m 
this group of plants are self-fertile, while the perennial 
forms are self-sterile. It would seem as if it were of more 
importance to ensure regular and uninterrupted produc- 
tion of seed in the former, while in perennials, even if m 
one season owing to the absence of insect visitors, seeds 
are not produced, the persistence of the plant to the next 
season when conditions may be m.ore favourable will 
enable it to reproduce its kind. 

The flowers of many ^'arieties of Apple and Pear are 
self-sterile, and disappointment has often been caused by 
the failure of a fruit tree to bear when grown singly with 



39 



no other variety near to it from which pollen could be 
carried by seed. E\'en m orchards, if a large number of 
self -sterile varieties are grown close together, the yield of 
fruit may not be found to be satisfactory. In both cases 
it has proved very efficacious to plant near such trees a 
Siberian or other Crab x\pple, which produces a large 
amount of effective pollen w^hich can be carried by insects 
from tree to tree. 

After fertilisation, when the young plant is beginning 
to be formed in the seed, the seed vessel too begins to 
swell, and it is from observing the latter that we can 
gather that pollination has been successful. Of course 
the continued grovth of the seed-vessel, as well as the 
development of the embryonic plant and the storage in the 
seed of enough food material for its subsequent germina- 
tion, necessitates considerable activity on the part of the 
vegetative organs particularly of the leaves. It is for this 
reason advisable in the case of Peas, Scarlet Runners and 
French Beans, when , they begin to set their pods, to 
encourage their further grov\th by the use of suitable 
stimulants such as liquid m.anure. 

It is obvious that as the formation of seeds and fruits 
require increased supplies of food material, herbaceous 
plants that are heavily fruiting will have less food where- 
with to develop new flovv^r buds, and graduallv the plant 
will cease to produce flowers. If, therefore, we are culti- 
vating plants for the sake of their flowers it cannot be 
too strongly recommended to pluck oft" all flowers as soon 
as they are dead, so that they should not begin to set 
their seeds. In the case of Sweet Peas, where the seeds 
take up a large amount of food miaterial, this is pa.r- 
ticularly important, but it is a good rule to follow in all 
cases. The fact that a large supply of food material to 
fruits may exhaust that available for the production of 
flower buds will explain reduction m the number of 
flxowers on fruit trees, if an abnormally heavy 
crop of fruit was produced m the previous season. 
It is not, how^ever, only the formation of flowers 
which will be interfered with. Under special cir- 
cumstances the vegetative organs, too, may suffer. 
For this reason it is recomimended that voung 
fruit trees, which have been recently transplanted, should 
not be allowed to ripen many of their fruits during the 
first season after transplantation, as this may interfere 



40 



with the proper development of the new root system, which 
is always damaged to some extent when trees are moved. 
Similarly, it is advisable when Raspberry canes are new^ly 
planted to cut them down to within eight or ten inches of 
the ground, so that they should not exhaust themselves 
during the first year, as that would prevent the formation 
of strong new shoots for the next season. 

As rifening fruits require a large amount of nutrition, 
it is important to see that the stimulating treatment we 
give to plants which are bearing, should not be used for 
the production of vegetative shoots. In Tomatoes the 
growth of such shoots should be checked v/hen the fruits 
are maturing. It is also advisable to reduce the foliage 
a little so that the fruits are not shaded. Excessive 
defoliation, how^ever, will prevent the fruits from attain- 
ing their full size as the leaves are the seat of formation 
for the organic material which the fruits require. 

When fully mature the seeds inside the fruits will be 
found to be surrounded with a hard seed coat, within 
w^hich we have the minute seedling either containing its 
store of food material in its fleshy seed leaves as in the 
Pea or Bean, or surrounded with a supply of nutriment as 
in the case of the Wheat grain. The food material is 
usually largely starchy or oily, but in addition there 
is a smaller am^ount of the essential nitrogenous material 
so important to the plant and so essential to man. Of all 
cereals Wheat is the richest in nitrogenous material, w^hile 
all leguminous seeds, thanks to the help which their 
parent plants have received from the nitrogen fixing 
bacteria in their root tubercles, have a great abundance 
of organic nitrogen compounds. This is the cause of 
their great nutritive value to mankind. 



Chapter 6. 



SEEDS AND SEEDLINGS. 

Vitality and longevity of seeds. Conditions favouring germi- 
nation. Seedlings. Variation. Natural and artificial selection. 
Sports or mutations. Hybrids and the laws of heredity. 

Ripe seeds are surrounded by a hard seed coat which is 
more or less impervious and prevents complete drying up 
of the embryonic plant within the seed. The more re- 
sistant the coat the longer the seed can preserve its vitality. 
The seeds of some plants germinate almost immiediately 
they are mature, hut most of them are adapted for and 
require a period of rest. During that time it is necessary 
to keep them dry and fairly cool. Under such conditions 
they can preserve their po\\'er of germination for some 
years, though a certain number even of resistant seeds v\ill 
die. It is also more difficult to germinate old seeds, owing 
probably to the drying of the seed coat, as well as to 
changes which have taken place in the living cells of the 
seed. When kept dry little alteration takes place 
internally, and die seed can remain dormant, practically 
no loss of matter taking place by respiration. For what 
length of time this suspension of anim^ation can last is not 
definitely ascertained, but we may say with certainty that 
we have no proof that seeds which have lain dormant for 
a thousand years or miOrCj like those which have been taken 
from izLgyptian miummy cases, can be germinated. That 
so-called mummy wheat and mummy peas have been 
originally obtained from Egyptian mummies must be re- 
garded as purely legendary. Accurate investigations 



41 



42 



which have been made regarding the longevity of seeds''^' 
have proved that so far a hundred years may be taken as 
the longest period for which seeds have been known to 
retain their powxr of germination. The seeds which enjoy 
such prolonged vitality belong to a restricted number of 
Natural Orders of which the Pea Family is one, remark- 
able like the others for the very resistant coat or testa with 
which its seeds are covered. 

After the resting period, seeds when placed in favour- 
able conditions commence to germinate. Moisture and a 
warm temperature javoiir germination. Water is absorbed 
by the seed coat as a whole, or may be taken in at certain 
points. In some small smooth seeds the outer layers of the 
seed coat are mucilaginous, and when whetted swtII up 
considerably and become slimy. I'his is the case with 
the seeds of the flax plant (linseed) and with those of the 
cress. The probable reason for this special provision is to 
cause the seeds to become fixed in the soil so that the 
seed leaves can be more readily withdrawn from the seeds. 
There are generally present on the inside of the seed coat 
certain layers of cells which swell up rapidly when the 
seeds absorb water, and probably aid in the splitting of 
the seed coat, thus enabling the embryo to be gradually 
withdrawn from the seed. Warmth, the other factor 
essential to germination, is required for certain important 
chemical changes which need to take place before germina- 
tion can be effected. The food material stored in the seed 
is largely of a solid nature and requires to undergo trans- 
formation so that it can be dissolved in the cell-sap and 
can be conducted to the growing root tip and to the 
developing leaves. By certain fermentative changes starch 
is converted into sugar, oil and organic nitrogenous com- 
pounds are broken up and pass from cell to cell. In 
some cases as in peas and beans the food material is stored 
in the two fleshy seed-leaves or cotyledons, while in other 
seeds like those of the melon, the onion, and in all our 
cereals it is found in cells outside the young seedling and 
requires to be absorbed by the latter before it can be made 
use of at the growing points. The re-awakening of the 
vital processes indicated by these internal changes is 
marked by the commencement of respiration, that indis- 
pensable accompaniment of all life, whether animal or 
* Ewart, A. J. " On the Longevity of Seeds. Proceedings of 

the Royal Society of Victoria, igoS. 



43 



vegetable. We can easily demonstrate the occurrence of 
this breathing- process in germinating seeds if we allow 
moistened seeds to germinate m a closed jar. The latter 
will soon lose all the oxygen it previously contained, and 
carbonic acid will be found to have taken its place, as can 
easily be seen by the fact that if a lighted taper is intro- 
duced into the jar it v;:ll be immediately extinguished. 
Respiration which is inseparable from active growth is a 
process of slow combustion, and is always accompanied 
by a rise m temperature. This can best be seen w^hen a 
large mass of seeds are germinating together as, for in- 
stance, during the process of malting, when barley grams 
m large heaps are passing through the early phases of ger- 
mination, the starch they contain in the resting stage being 
transformed into the sugar known as maltose. By plung- 
ing the hand into a heap of barley which is undergoing 
this change one can readily detect the heat which is being 
evolved. The active need for oxygen by germinating 
seeds w^ill make us careful to keep our seed beds porous 
and to prevent the soil above seeds from becoming caked 
and therefore impervious to air. 

Details of the various methods of germination will be 
found in most botanical textbooks and need not be dealt 
with in this lecture. I might, however, micntion that whilst 
most seeds germinate best in the absence of light, there are 
some small seeds which should be sown superficially, as 
light seems to be beneficial to their development. This is 
particularly the case with the seeds of some grasses. An 
important point in sowing seeds, apart from taking care to 
give them the proper depth, is to ensure that the soil with 
which they are covered is fine and friable, so that the 
seedlings have no difficulty in forcing their way up to the 
light. In the case of mustard, cress and other members of 
the same famnly (Cruciferae), care should be taken not to 
over-water the seedlings as they are liable to damp)-oft^ " 
owing to the attacks of a parasitic fungus, which will be 
dealt with in a subsequent chapter, \\lien dealing with 
some seeds which germinate irregularly, possibly owing to 
their age or on account of their having been stored during 
the winter when they should have been sown immediately 
after maturing, it may be advisable to keep the seedpan 
going for a considerable time as a succession of seedlings 
may be produced, all of v-hich may be quite healthy and 
normal. 



44 



Let us now pass on to consider the nature of the seed- 
lings themselves. It was pointed out previously that 
Darwin had shown the beneficial effect of cross-pollination 
m the more vigorous development of the offspring. Another 
important feature of seed reproduction as compared with 
vegetative reproducton, particularly if the seeds are 
the result of cross-fertilisation is the occurrence of a con- 
siderable amount of variation. The more dissimilar the 
parents the more varied are the offspring, and the greater 
therefore the scope for the play of natural or artificial 
selection. 

The term variation has been used for two very different 
phenomena noticeable when examining a large number of 
plants or animals of the same species or kind. It is well 
know^n that the offspring of any two parents all have some 
different individual characteristics, and a close observation 
of a number of seedling plants will show us that though 
they all have a general resemblance, we find that they 
differ slightly one from another in size, in the shape and 
texture of their leaves, and when they grow up in colour and 
conformation of their fiovvers. Indeed, if we had carefully 
examiined the seeds from which they have grown we should 
have found that the latter showed already a considerable 
range of variation in shape and size, and possibly also in 
colour. Such slight individual variations are always 
found to fluctuate around a m.ean or average which we 
may look upon as the general character of the species or 
race. It is these slight individual variations which 
Darwin regarded as so important in the evolution of new 
forms, those least suited to the particular conditions of 
life gradually dying out and leaving those which were fit 
to survive. l\[atural selection operating in this vv'ay was 
thought to have produced the innumerable forms w4iich we 
know as natural species. In the same way man, by making 
a choice of the plants most suitable to his purposes, has 
by artifixial selection produced the strains and varieties 
now cultivated. 

Recently experiments have been carried out in which 
the seeds of certain plants have been carefully graded and 
the largest sown separately with a viev/ to ascertaining 
whether by such constant selection the seeds could be in- 
definitely increased m size. This was, however, not found 
to be possible, for though the average size of the seeds w^as 
at first considerably raised, a limit was reached beyond 



45 



which it was impossible to increase the size of the g-rains. 
By this process of selection the investigator had probably 
succeeded in isolating a pure strain of the particular 
variety with which he was experimenting, characterised bv 
a larger seed than the sample with which he started, which 
was, no doubt, a mixture of races, some producing smaller 
and some larger seeds. But once a pure line had been 
obtained, though it showed slight fluctuations, yet it could 
not be improved by selection of die extremes of these 
fluctuating variations. 

We know, however, of other variations which arise from 
time to time in all species of plants, and which are very 
different from the fluctuations as wx may term those 
described above. Sometimes we come across new^ forms 
which differ considerably from, the normal type in one or 
more characters, as, for example, the cut-leaved varieties of 
so many plants. In this case we do not iind a series of 
forms intermediate between the cut-leaved individual and 
those with normal foliage. This second type of variation 
which arises suddenly Darwin called a sport, and he con- 
sidered it to be of comparatively little importance in the 
evolution of plants, as its very infrequent occurrence would 
cause it to disappear m nature by constant inter-crossing 
with the more numerous normal forms. By artificial selec- 
tion, however, man can perpetuate and establish these 
sports, as has obviously been the case with many of those 
forms which took the fancy of the horticu'lturist. It is to this 
kind of variation that w^e owe so many of our interesting 
and peculiar forms of cultivated plants. Recently a Dutch 
botanist, De Vries, has endeavoured to show that this 
second kind of variation, which he calls viutaiiun, to dis- 
tinguish it from the former or fluctuating variation, is of 
more frequent occurrence than had been supposed by 
Darwin. He observed that a certain large group of plants 
of Evening Primrose which had established itself in a wild 
condition in Holland showed a very considerable amount 
of mutation, and his expeiiments and other observations 
led him to the conclusion that at certain periods, possibly 
owing to changed environment, plants passed into a phase 
of mutation, during which numerous new sub-species or 
races might arise. Interesting and important as JDe Vries' 
experiments are, his case cannot be considered proved until 
we know more about the previous history of the plants 
which show^ such considerable mutation. At present we do 



46 



not know whether the Evening Primrose which he investi- 
gated was a pure race or of hybrid origin. 

To horticulturists a knowledge of hybrids is of the 
greatest importance, a vast number of new and interesting 
forms being continually produced through hybridisation. 
.\t one time it was thought that hybrids were invariably 
intermediate between the two parental forms, and that they 
were generally sterile, and could therefore not be repro- 
duced by seed. Though this is sometimes the case,, 
particularly where the parents are of different species, it is 
by no means the rule, and certainly not in the case of 
hybrids between two different varieties. Our exact know- 
ledge of hybrids dates from the careful experiments made 
in the latter half of the nineteenth century by Gregor 
Mendel^ of Briinn. Unfortunately, his important investi- 
gations did not becom^e generally known until die begin- 
ning of the present century. ]\Ienders first observations 
were made on various varieties of the garden pea, and he 
obtained the striking result that in crossing two different 
strains the offspring were not of immediate type, but 
generally inherited the characters of one of the parents in 
their entirety. Thus in crossing varieties, of which one 
had round and smooth and the other wrinkled seeds, he 
obtained seeds all of which were round. He therefore 
considered this to be a dominant character, while he called 
wrinkledness recessvve. But when the flowers of the hybrid 
plant were subsequently fertilised with their own pollen, 
the seeds they produced were not all round, some of themi 
were wrinkled like those of one of the grand parents. On 
carefully counting the number of these recessive types, he 
found that one m four had reverted to the wrinkled tvoe. 
Aloreover, he found that of the round seeds some were of 
pure type, and when further cultivated always produced 
round seeds, while others were of hybrid nature and these 
always produced reversions to the parents which had been 
originally crossed. 

He was able, finally, to demonstrate that of the off- 
spring of every hybrid when self -fertilised one quarter 
reverted completely to the female parent, one quarter to 
the male parent, while half of the offspring remained of 
hybrid nature. These resembled the parent, which pos- 
sessed the dominant character but preserved the recessive 
character in a dormant or latent form as was shown by its 
reappearance in a subsequent generation. The accurate 



47 



numerical results, which Mendel obtained, can be readily 
explained on mathematical grounds by what we know of 
the various combinations which are possible between egg- 
cells and pollen-grains when differing in two characters. 
Of course, If the parental forms dift^er m several characters 
the possible combinations are much more numerous and the 
numerical chances of complete reversion to the parent forms 
IS much smaller. On the other hand, w^hile w^e obtain a 
large number of hybrid forms which will show reversions 
to the original parents, we shall find some new combina- 
tions which are pure forms and therefore breed true. These 
we can isolate from the rest by the rejection of all strains 
w^hich show reversion, and thus we can obtain new^ and 
permanent varieties. This no doubt has been the means 
adopted by plant breeders in the past from practical 
experience, but thanks to Mendel and those who have fol- 
lowed up the path shown us by his investigations, a scien- 
tific basis has been laid to the practice of hybridisation. 

In the course of these scientific enquiries some remark- 
able facts have come to light. It has, for instance, been 
discovered in crossing a w^hite and a yellow variety of the 
^slarvel of Peru (Mirabilis) that the hybrid produced was 
not intermediate in character, that is, of pale yellow 
colour, neither was it like either of the parents, but of a 
pink colour and marked with red stripes. The white form 
must evidently have possessed some chemical factor w^hich 
changed the yellow colour into red, while the character 
producing striping, which could not show itself in the 
white form, became visible in the hybrid by reason of the 
coloured sap. The offspring produced by self -fertilisation 
of this hybrid were of tw^elve different kinds : five with 
different shades or striping in yellowy five corresponding 
forms in red, and tw^o white forms, which, though re- 
sembling each other externally, differed in constitution as 
could be seen from their progeny. 

A somewhat similar and equally remarkable result was 
obtained by crossing two w^hite Sweet Peas belonging to 
the variety, Emily Henderson, the offspring having a 
partly-coloured flower, red with blue keel, probably very 
like the ancestral form. Of the two white forms evidentlv 
each one contained a special factor, which combined with 
that of the other white form caused the formation of a 
coloured sap. The offspring of the coloured hybrid were 
mostly coloured but in different degrees, while white form.s 



48 



similar to the two original parents were also produced, the 
two colour-producing factors separating out in these par- 
ticular descendants. 

Though in a comparatively short space of time great 
advances have been made in our knowledge of the laws 
of hybridisation and heredity, miuch remains still to be 
learnt in this important branch of scientific knowledge 
and the co-operation of the plant breeder with the scientific 
investigator is much to be desired. The success alreadv 
attained in this country is of good augury for the future. 

Further information on some of the subjects discussed 
in this lecture can be obtained from the following 
books : — 



Dar^.vin, C. *' Origin of Species/' and ''Animals and ^ Plants 
under domestication/'' 

De Vries, H. '' Species and Varieties, their Origin by ]^\[utation.'' 
Chicago, 1905. Also ^' The' Mutation Theory/'' London, loog. 

Bateson, AV. ''•' Alendel's Principles of Heredity/' Cambridge 
University Press^ iQog. 

Punnett, R. C. " ]\Iendelism/"- Macmillan & Bowes, Cambridge. 
1Q05, i/-. 



Chapter 7. 



MALFORMATIONS AND INJURIES. 

Malformations arising as sports or by malnutrition. Healing 
of wounds. Injuries due to lightnings frosty etc. Harmful 
effect of smoke and fog. 

Abnormal development of various parts of plants 
whether of vegetative or of reproductive organs may be 
regarded as ^nalforinations. Many of these arise as sports 
or mutations^ and may be transmitted to the progeny of 
the plant. In the case of the vegetative organs such occur- 
rences as pitcher-shaped leaves have been noted in many 
instances. Leaves of this type are of no special advantage 
to the plant, they may indeed be a disadvantage, causing 
water to collect on the leaf surface and thus increasing the 
liability to fungal attacks. Among ferns an excessive 
development of the margin of the frond leads to the 
development of so-called crested varieties, which being 
pleasing to some tastes, have been perpetuated by artificial 
selection. We have no knowledge of the causes of such 
excessive development of the leaves, nor are we certain 
of the origin of similar unnatural developmxcnts in stems, 
such as appear in the case of " fasciation (from the Latin 
fascis^ a bundle). This term is applied to the abnormal 
development of the stem into a broad flattened structure, 
often caused by the fusion of several stems, or by the union 
of lateral shoots with the main stem. Superabundance of 



49 



50 



food supply may in some cases be responsible for this 
abnormal growth. Sometimes these forms occur spon- 
taneously without apparent cause, and have been cultivated 
by horticulturists, as in certain species of Echinocacttis 
and in the flowering shoots of the Coxcomb [Celosia 
cristata). 

Almost all cultivated plants are known to produce 
occasionally variegated leaves, and in some cases leaves 
which are entirely devoid of green colour. In some in- 
stances these have been traced to lack of nutrition. We 
know of course that if seedlings are raised in the dark or 
potato tubers are allow^ed to sprout in the absence of light, 
the leaves will be of a sickly yellow nature, as light is 
essential for the production of the green colouring matter 
characteristic of foliage. Similarly, absence of iron salts 
in the soil will prevent the formation of chlorophyll. It 
has also been noticed that young shoots arising below a 
graft m the case of hollies are often white in colour. This 
must be regarded as due to an mterference in the dow^n- 
ward conduction of material caused by the artificial union 
of the tissues at the graft. Some cases are also known 
of wild plants with variegated foliage, which under culti- 
vation in richer soil have developed normal green leaves. 
It has also been possible by growing plants in the green- 
house at higher temperatures to change the variegated 
into green leaves, and to prevent the formation of further 
variegated leaves in the case of some plants. 

Some botanists have considered that a special substance 
or virus is developed in certain parts of a leaf which 
has prevented the formation of the green colouring matter; 
but as we do not know anything of the nature of such a 
substance it is perhaps simpler to consider that the chloro- 
phyll granules in certain portions of the leaf have re- 
mained in a more youthful condition, in w^hich they pro- 
duce the pale yellow colour w^hich alw^ays precedes the 
green colouring matter. Apparently this stoppage in the 
development occurs more commonly in plants under culti- 
vation than in their natural condition. Variegated leaves 
are obviously less efficient as nutritive organs, and varie- 
gated plants are therefore often less resistant than normal 
forms. Their foliage is more easily affected by heat or 
frost and the leaves are less long lived. 

There are some cases in which variegation is considered 
to be a disease produced by bacterial action. The so- 



51 



called ''mosaic disease" of Tobacco plants and of 
Tomatoes is supposed to be due to this cause, and certain 
experiments indicate that it is highly infectious. 

Most gardeners will have come across examples of 
abnormal formations in flowers. Often the sepals may 
become leafy as in Jack-in-the-Green Primroses, sometimes 
the petals have a leaf-like appearance as in the Green Rose. 
To the flower-lover these transformations are more curious 
than beautiful, but to the botanist they are of considerable 
interest as cases of reversion. For we mxust assume that 
the various floral organs are all modifications of leaves 
which have become adapted to the special function of 
reproduction, and in such f oliaceous developments we may 
see a retrogression to a more primitive type of leaf. More 
rarely, but still occasionally, we may find stamens or pistil 
becoming transformed into vegetative leaves; the latter is 
often the case in the flowers of the Double Cherry. In both 
cases sterility of the flower is caused. In other sports the 
outermost leaves of the fiower, the calyx, may become 
coloured and delicate in texture like the corolla. This is 
the condition in the hose-in-hose variety of the Polyanthus. 
More frequently it is the inner leaves of the flower, the 
stamens, that becomie petaloid. The " doubling " of 
fl.owers is sometimes found to take place in wild plants 
when transplanted into garden soil. In all probability 
the tendency to doubling is independent of the cultivation 
of the plant, but the rich nutrition which the plant receives 
accentuates the efiect. Certainly double varieties to be 
kept in perfect condition require an abundant supply of 
food, and are liable to degenerate when grown in poor soil. 
Degenerate is perhaps not a good term to use, as the plant 
is in a more perfect condition from the reproductive point 
of view when the stamens and pistil are normally 
developed and fertile than when they are transformed into 
showy petals. In that case the flower is generally com- 
pletely sterile, though it may sometimes retain a few 
seiviceable stamens or a receptive seed vessel. 

l^rom the doubling of flowers such as the Rose, we ixiust 
distinguish the doubling of the so-called " flowers " of 
Daisies, Chrysanthemums, and other members of the Com- 
posites. In this Family the apparent flower is really a 
head of small flowerlets or florets closely crow^ded to- 
gether. These are often of two kinds, showy ray florets 
and small central clisk-florets usually of a different colour. 



5-2 

When these "flowers'' are said to double, it is by trans- 
formation of the tubular flowers of the centre into con- 
spicuous strap-shaped fio\\-crs, like those of the margin of 
the inflorescence. In that case they need not necessarily 
become sterile, for m many Composites like the Dandelion, 
all the florets are normally of the ray-fl.oret type. Another 
curious freak inec with m some of the Compositse when 
they receive abundant nutrition is the development of some 
of the ray-florets into small heads of flowers on stalks of 
their own, so that the large central head is surrounded by 
a number of lesser ones. I'his is the condition in the 
Hen-and-Chicken Daisies, and similar modifl.cations are 
found in other plants. 

A not uncomimon abnormality m gardens is the 
development of a terminal flower of large dimensions and 
regular shaped at the top of the flowering spike of the 
Foxglove; in the Snap Dragon, too, flowers showing radial 
symmetry instead of the usual two-lipped condition may 
be formed. A plant may indeed bear nothing but 
peloric " flowers as these are called. 

[Monstrosities may also occur in the development of 
fruits. Double-fruited oranges with two whorls of carpels 
;pegs) one inside the other are occasionalh' found, while 
in one variety known as Buddha's Angers, the various 
carpels are only united below and taper off above into 
finely divided pocl-like segments. 

\\liile most of these m_al formations are due to little 
understood intn-nal causes, many cases of excessive devel- 
opment of plant structures are produced as the response 
to irritation. Thus, numerous kinds of galls and tumours 
may be developed by animal and vegetable parasites, 
insects and fungi, vs'hich injure the tissues and cause them 
to swell or grow more vigorously. These will be dealt 
with m later chapters. 

Plants like animals have the power of protecting them- 
selves against external iniury and of healing any wounds 
that are caused thereby. The most usual method is by the 
formation of layers of cork, which will protect any exposed 
part from the atmosphere, thereby preventing the excessive 
loss of moisture through the wounded surface and also 
reducing the chances of invasion of the tissues by disease 
or decay producing organisms. This development of a 
resistant layer of cork is due to the active growth of the 
living cells of the plant in the exposed region. This active 



53 

growth requiring energy, which plants gam by their 
respiratory process, we find that the heahng of wounds 
IS marked m plants as m annnals by a local rise in tem- 
perature. 1 he wound tissue which is developed at first 
IS soft and termed callus; it is that excrescence of cells 
which IS produced as we have seen around the base of a 
shoot in making cuttings. Within this callus layer imper- 
vious cork is then formed and this is usually sufficient for 
the healing of herbaceous plants or soft tissues. In trees 
and shrubs, however, if the stem or branches are deeply 
cut, the wound will subsequently be covered up by woody 
layers as well. The formation of these liquified tissues 
commences at the margin of the wound and they gradu- 
ally cover over the entire wounded area. It is thus that 
branches broken at their base become covered up in nature, 
and are found as knots buried in the wood. In cultivation, 
of course, m.uch larger branches are often amputated than 
usually break off m nature, and though their stumps will 
ultimately be covered up, it takes a considerable time in 
the case of a thick branch, and before the wound is closed 
up there is plenty of time for the wood exposed by the 
cut to commence to decay. To prevent this from taking 
place, it is advisable to coat the surface of the stump 
immediately after the removal of the branch, vnth tar or 
some antiseptic substance, which will prevent the entrance 
of bacteria or of other harmful fungi. 

Iniitry by ligliining, if it is deep and considerable, may 
be beyond the power of the plant to repair; but where 
only the outer layers have been damaged, in vvhich case 
a single longitudinal fissure is generally found, running 
vertically down the side of the stem, callus and cork 
usually ileal up the gap completely. 

Frost is ill all probability one of the most frequent of 
external factors causing injury to plants. Though we 
cannot as a rule prevent the havoc wrought b>' frost, it is 
not without interest to note what is the efi'ect of freezing 
upon plant structures. It is particularly the young grow- 
ing parts of plants winch are nipped bv frost, wliile the 
mature leaves are often undamaged. On the other hand, 
somewhat fieshy plants like the Xasturtium, do not resist 
frost very well. The cell sap containing a good many 
organic acids and often sugar in solution, does not freeze 
readily. The denser the sap is, the less danger there is 
of its being frozen. As a protection against the effect 



54 

of frost, we find that with a lowering of temperature some 
of the \vater from the cell-sap passes into the spaces 
which exist between the cells in mature tissues. Thus the 
sap becomes more concentrated and less liable to injury, 
while the water which may become frozen in the inter- 
cellular spaces does no damage if the latter are of fan- 
size. If they are sm.all, then the expansion of the water 
m freezing may tear the tissues asunder and thus injure 
them. In the growing parts of the plant there are tiny 
intercellular spaces, so that little water can be passed 
out of the cells, and consequently the sap itself may be 
frozen, m which case the protoplasm, the living substance 
of the cells, is killed and the tissues become blackened. 
A very sudden frost is always more harmful than a long 
spell of frost preceded by a gradual lowering of the 
atmospheric temperature, as plants are able to prepare 
themselves by a condensation of their cell sap. It is the 
same with recovery from frost. Plants in a frozen con- 
dition may recover readily if they are slowly thawed. 
AVhen exposed to bright sunshine on a frosty morning they 
may be perm.anently injured as often happens with Wall- 
flowers. It IS therefore v\'ell to shade frozen plants from 
the direct rays of the sun, so that they are thawed less 
rapidly. We must remember that m bright sunshine the 
pores of the leaves open and the plants are rapidly trans- 
piring, and if this takes place while the ground is still 
frozen and the roots are unable to absorb water, the con- 
sequences may be serious. 

Those of us who have laboured in town gardens are 
only too familiar with disappointment due to general sick- 
liness of some of our plants and to the Jiarvifiil effect 
of a Siiioky atmosphere. Worse m winter than in summer 
it is nevertheless pronounced even in the latter, and records 
taken show that there are quite a number of gloomy days 
in July and August, and that air pollution is quite appre- 
ciable during these months. From actual measurements 
made during the month of July, it has been calculated 
that if we draw a circle of a mile radius around the Town 
Hall of ilanchester, 195 tons of impurities would be col- 
lected during the month from this area, and of this 
100 tons would consist of soot or other insoluble miatter r'' 
- See First Annual Report of the Sanitary Committee of the City 

of ^Manchester on The Work of the Air Polkition Advisory 

Board."' I\Iarch, iQiS- 



55 

and ^lanchester is by no means the worst town in this 
respect. It is gratifying to know that efforts are being 
made in many of our industrial centres to understand, and 
let us hope, also to cope with this problem of air pollution, 
and all such movements deserve the hearty support and 
co-operation of gardeners and ilower-lovers. 

From what has been said in earlier chapters, it will be 
clear that bright sunshine is the most important factor 
in the nutrition of plants, as it is only in the presence 
of light that the green chlorophyll of the lea\'es is able 
to form starch, the food so essential for the further growth 
of the plant. A murky atmosphere therefore lowers con- 
siderably this prime nutritive activity of the leaves. If 
in addition to that the leaf, as is always the case with 
town evergreens, has become coated Vv'ith a more or less 
opaque layer of sooty matter, the normal power of leaf 
nutrition is still further reduced. What wonder then that 
our town and suburban gardens suffer many serious losses. 
It is only hardy plants which can survive these adverse 
conditions. Conifers are usually regarded as fairly 
resistant plants, and so they are to drought or cold, but 
smoke is most harmful to them as their breathing pores or 
stomata are situated at the base of a depression on the 
surface of the leaf, and this pit becomes partiallv filled 
with soot and the pore is thus blocked. In ordinary leaves 
these pores are usually on the protected under surface and 
are therefore not endangered by the smoke, but m the case 
of the needle-shaped leaves of the conifers they are freely 
exposed to smoke and fog. As a consequence we nnd 
that conifers do not thrive in the neighbourhood of our 
industrial towns. A blackening of leaves is, however, not 
the only noticeable effect of air pollution on vegetation. 
We observe, particularly on evergreens, that many of the 
leaves have brown spots or dead margins, and in both cases 
this is traceable to the acidity of the atmosphere, which 
kills the tissues, particularly those near the stomata or 
breathing pores. It is particularly in ivinier fogs that 
the air becomes very acid, owing to the sulphur contained 
in the coal, and our own sense organs enable us easily to 
detect the presence of the sulphuric acid. Even in summer 
the acidity of the atmosphere causes the discolouration 
and the early fall of the leaves of trees in our parks and 
town gardens. 



56 



It IS obvious that the rain washes this acid into the 
soil, so that not only the leaves but the roots also suffer 
from its effect. Ram water collected in or near our tow^ns 
is always slightly acid, and a great difference can be 
observed in the growth of seedlings, some of which are 
watered with ordinary rain water and others with rain 
v/ater in wdiich the acid has been neutralised. A consider- 
able number of striking experiments on this point have 
been made by Mr. Ruston, of the Leeds University. The 
lesson they teach us is that the soil of gardens or allot- 
ments near our large towns should be well treated with 
lime, so as to neutralise as far as possible the acidity which 
is caused by the rain washing the smoky atmosphere. 
A consideration of the fact that this smokiness and acidity 
of the air is greater in winter than summer suggests that 
a dressing with lime in the spring before growth recom- 
mences would be especially beneficial. Experience has 
also taught us that many biennials are better wintered 
under glass than when left in the open, e^'en when they 
readily withstand the frost. A glance at the condition 
of the lights of a cold framie will show us what a coating 
of soot we have prevented from collecting on the leaves. 
A really penetrating fog, however, will get into frames 
and greenhouses, and may often do considerable damage 
to the delicate blooms of Orchids and to many other 
flowers, besides causing in some plants the leaves to become 
discoloured or to fall.* 

See F. "W. Oliver on " The Effects of Urban Fog on Culti- 
vated Plants." Journal of the Royal Horticultural Society, 
vol. xiii, 1 891. 



Chapter 8. 



FUNGI AS A CAUSE Of DISEASE IN PLANTS. 

Common fungi, mushrooms, toadstools and moulds. How they 
live. Saprophytes and Parasites, The ''Damping off disease 
of Seedlings caused by the fungus Pythium, Life Story, means 
of spreading and living in the soil, precautionary measures. 

In preceding chapters you have become acquainted with 
the normal uses of the parts of healthy green plants,. We 
are now to consider plants in disease and especially the 
disturbances of the structure and functions of plants pro- 
duced by parasitic fungi. It is obviously impossible 
within the limits of a few chapters to deal with the whole 
subject of diseases of plants caused by fungi, but by a 
consideration of some of the diseases most common in 
fields, gardens and greenhouses, the main features of the 
subject may be illustrated. By this treatment you will 
not only become more familiar with the causes of these 
particular diseases, but also with the sort of precautions 
and remedial measures it is worth while to adopt in rela- 
tion to the various types of plant disease. 

Practically all parts of plants are subject to disease. 
In some cases we have immxediate and complete destruc- 
tion of the plant or part attacked, but in others the death 
may be delayed or the plant may support the parasite in- 
definitely. We have diseases of seedlings, diseases of 
roots, of stems and of leaves, and a very large class of 



57 



58 



plant diseases comprises those which attack fruits. In 
this lecture we shall consider the most common disease of 
seedlings which is known as the ''Damping off" disease. 

Before dealing in detail with this, however, let me 
recall some of the more essential features in the structure, 
nutrition and life history of the more common fungi, facts 
with which you are no doubt, to some extent, already 
familiar. In any of the fields and lanes around our large 
towns, especially in autumn, it is possible to find examples 
of the larger fungi such as mushrooms and toadstools. 
Whilst the colour and form of the part which we see above 
ground are often striking, in reality this is only the repro- 
ductive part of the toadstool or mushroom plant, just as 
much as the flower and fruit are the reproductive parts of 
the higher plant. It is true that the mushroom as we 
generally see it, appears above ground, sheds its spores 
and decays all within a few weeks; but the vegetative part 
of the plant lives in the ground for a very considerable 
time before it enters on the reproductive phase. 

The bricks of so-called mushroom spawn contain 
quantities of fine interlacing threads of fungus in manure 
which are really the vegetative part of the mushroom. 
Under the micro'scope these filaments are seen to consist 
of long branched tubes. These tubes are divided up into 
chambers or cells by cross partitions, and each cell is 
lined with the jelly-like semi-transparent living substance, 
protoplasm. Within this are drops of water and oil as 
well as certain denser granules, but filaments of fungi 
never contain the green colouring matter found in higher 
plants. The protoplasm is the living part of the cell, 
and food material is taken in from the soil or manure 
through the protecting cell membrane. Such food 
material often appears stored in the tubes as drops of oil. 

Although the filaments of the vegetating mushroom 
plant are not bound together into a complex plant body 
like the cells of flowering plants, yet the loosely inter- 
woven threads behave as a whole, and after weeks or 
months of vegetative growth give rise to the definite fruit 
bodies consisting of numberless aggregated filaments. 
These reproductive bodies usually possess a stalk bearing 
an expanded cap from the under side of which project the 
radiating gills. Upon the surface of the gills are borne 
large numbers of minute round or oval spores. Each 
spore is really a single tiny cell so small that it can only 



59 



be seen if very highly magnified. If, however, the stalk 
is removed from a not over-ripe mushroom, and the latter 
pla-ced on a sheet of white paper, in the course of a few 
hours so many spores are shed from the gills that a print 
of the gills is produced on the paper. This print consists 
of myriads of spores which have fallen like dust from the 
gills. The spores are the reproductive cells of the fungus 
and, falling to the ground, they grow out to produce tiny 
filaments. They soon begm to absorb water and food 
substances from decaying vegetable matter in the soil and 
manure, and so the vegetable life of the plant is carried 
on. 

Many fungi are much more minute than the mush- 
room, nideed are so small that they can only be pro- 
perly studied under the microscope. As an example of 
such a fungus we have Mice or, the common white mould 
which often appears on damp bread or dung. If a piece 
of bread is kept moist for a few days under a bell-jar 
in a w^arm place the mould soon appears as a dense 
growth of fungus covering the surface of the bread. Erect 
silky threads stand up from the surface like a miniature 
forest. Microscopic examination shows that the fungus 
consists of two sorts of filaments : fine ones w^hich branch 
and ramify in all directions forming a felt on and in the 
substratum, and the coarser erect ones which stand free. 
The finer netw^ork on the substratum is the vegetative part 
of this fungus, while the erect coarser aerial threads are the 
reproductive organs. The individual branching filaments 
are very similar to those described for the mushroom, ex- 
cept that in this case there are no cross walls dividing up 
the tubes. When the aerial filaments ha^^e grown for a few 
days there appears at the extreme tip of each a minute 
round swelling like an inflated ball about the size of a pin 
head. The protoplasmic contents of this globular body or 
spore-case soon become divided up to form a large number 
of spores, and then it has the appearance of a miniature 
ball full of shot. As the wall of the spore-case becomes 
dry it breaks, scattering its spores into the air as a fine 
dust in all directions; the spores of Mucor and, indeed, 
of many fungi, are very light and easily carried by the 
slightest current of air. When placed in water the spores 
germinate in a few hours, the protoplasm within absorbs 
water, the spore wall bulges in one place and grows out 
forming a fine filament which, given suitable food 



6o 



material, rapidly grows and branches to form a dense felt 
such as we saw on the moist bread. 

We have seen that suitable food material for this 
fungus is provided by bread, dung, etc.; that is to say by 
dead or decaying plant or animal matter, just as the 
mushroom or toadstool lives on similar dead organic 
remains m the ground. Fungi, which in this way draw 
all their nutriment from the rotting remains of plants or 
animals, are known as saprophytes, and although they 
are unable to injure living plants they readily feed on 
their remains after death. 

Fungi, unlike green plants, possess no chlorophyll 
and are therefore unable to construct their own carbon 
compounds such as starch and sugar from the carbonic 
acid of the air. They, however, take in such complex 
organic substances ready-made from the remains of plants 
which have previously m.anufactured them, and saprophy- 
tic fungi play an important part in Nature in living upon 
and decomposing the dead organic remains of plants and 
animals. The saprophytes as a rule cannot attack living 
plants, and therefore do not give rise to plant diseases. 

A large number of fungi, however, are unable to live 
even upon decaying plant remains, and derive nutriment 
from the cells of living plants. Such fungi are parasites, 
and not only do they require to take m the carbon com- 
pounds of their food material ready-made, but they can 
only take their food substances from living cells. Now 
it is clear that in obtaining substances forcibly from plants 
while still alive, such parasitic fungi rob the plants 
attacked of materials which otherwise would have been 
used for their own life and growth, and may harm them 
more directly m doing so. A plant which harbours a 
parasitic fungus is spoken of as the host plant, and in 
m.ost cases the host suffers injury not only because it is 
robbed of substance by the fungus, but also because the 
work of the particular parts of the plant infected are 
seriously interfered with. shall see for example how 

the stores of food material in roots like turnips, or tubers 
like potatoes, are raided by fungi, and on the other hand 
how mildews and rusts prevent leaves from manufacturing 
food supplies. 

]\Iost of the larger and more prominent fungi are sapro- 
phytes and • live, as we have seen, on decaying organic 
matter. Some of them, however, are parasites. The 



6i 



bracket fungi and the honey agaric, for instance, live as 
parasites on trees. The vegetative filaments of these 
fungi ramify in the tissues of the tree, gaining entrance 
often through wounds, and only the reproductive bodies 
are produced outside. But very many diseases of plants 
are caused by fungi which, like the saprophytic mould 
Mucor^ are so small that they can only be studied with 
the aid of the microscope. Such a minute parasitic fungus 
called Pythimn de Baryanuin often causes the "'Damping- 
off " of seedlings. 

The 'VDamping~off disease very frequently appears 
when the seeds of many plants are sown too thickly and 
grown under conditions which are too warm and moist. 
Young seedlings begin to die off in patches and soon 
present a very characteristic rotten appearance. The disease 
is very commonly met with in gardens and greenhouses, 
occurring in seed-beds of all kinds a few days after the 
germination of the seeds. It is most abundant in very 
wet weather and when the beds are kept too shaded and 
badly ventilated; crowding of seedlings also favours the 
progress of the disease. At first a few^ individual seed- 
lings are attacked at or near the surface of the ground, 
the tissues in this region having a water-soaked appear- 
ance. Soon the cells collapse, and being unable to 
support the weight of the cotyledons the seedlings 
fall prostrate. Those immediately around are similarly 
affected, the disease spreading through the seed-bed m 
ever widening circles until practically the whole may be 
destroyed. When the plants fall over they become pale 
and rotten, and soon the whole bed is reduced to a moist 
mass of decaying seedlings. This mass is seen, on closer 
examination, to be covered with the very delicate threads 
of a fungus somewhat like the mould on bread. Often 
the filaments can be seen to have spread from the first 
diseased seedlings to the outer parts of the circle. These 
filaments most often belong to the fungus Pythium, which 
is so called because of its ability to produce rotting. The 
fungus continues to grow in the dying seedlings, and the 
filaments may form a dense felt over the whole seed-bed. 

If a seedling is examined under the microscope shortly 
after it is attacked, the collapse of the tissues just above 
the ground is seen to be due to the destruction of the 
cells at that spot. This destruction is caused by the fila- 
ments of the fungus w^hich at this stage are seldom visible 



62 



to the naked eye. They are, however, almost exactly like 
those found on the mouldy bread, and they run both 
between and w^ithin the dead and dying cells. They can 
also be traced into and between adjoining living cells, 
and when the seedling falls prostrate the remainder of 
the plant is soon invaded, for it is kept moist by contact 
with the damp soil. As the tissues further decay the 
filaments of the fungus spread out over the soil reaching 
across to other seedlings which then succumb in the same 
sort of way. The fungus attacks the cells of the seed- 
lings by first extracting w^ater, then boring through the 
cell walls and finally killing the living protoplasm and 
feeding upon the cell contents. Since the fungus growls 
very rapidly a seedling may be reduced to a putrid mass 
in a few hours. 

The life story of Pythium is typical of the most 
thorough and destructive of plant parasites. The fila- 
ments at first grow in the air spaces between the cells, but 
later they enter and kill the living protoplasm. The 
spread of the fungus at first is due to the growth over the 
soil from its earlier victims to healthy seedlings which 
are attacked in turn; but soon a more rapid means of 
spreading comes into play. When a seedling has become 
thoroughly infected by the filaments of the fungus the 
ends of many of the branches of the latter begin to swell 
out into globular bodies very like the spore-cases of Mucor, 
only much smaller. These sw^ollen bodies are full of 
protoplasm and serve a somewhat similar purpose to those 
in Mucor. They are really special spore-cases, but in 
further development they dift'er considerably from those 
of Mucor. In that fungus the spore-cases w^ere borne on 
filaments standing erect from the surface, the spores being 
shed into the air. In Pythium, however, the spore-cases 
are submerged in the film of moisture around the decay- 
ing seedling, and the different behaviour probably depends 
on this. When the spore case is mature a short tube grows 
from the side and swells to form a globular body with a 
very thin wall. Into this the whole of the protoplasm 
passes from the spore-case ; it then rapidly divides into 
9 or lo small rather oval masses of protoplasm, which 
begin to writhe and wriggle wathin the thin vesicle. This 
soon bursts, liberating the minute writhing spores which 
swim about in the slight film of water on the soil or sur- 
face of the seedling. When examined under high mag- 



63 



nification each of the colourless swimming spores is seen 
to be furnished with two excessively fine hair-like threads 
of protoplasm which, by lashing the water incessantly, 
bring about the movement of the spore. The active move- 
ments continue for twenty minutes or half an hour, then 
the spore comes to rest, rounds off and withdraws the 
whips of protoplasm. If favourably situated on a seed- 
ling it sends out a fine lilament which bores its wa}' 
through the outer wall of one of the cells and grows into 
the interior. It has been proved that the tip of the fila- 
ment is able to do this, because it secretes a substance 
which enables it to digest its wa}' through cell w^alls 
pretty much as gastric juice renders our food materials 
soluble. Once within the cells of the victim the fungus 
branches and grows rapidly from cell to cell, spreading 
destruction as it goes and deriving nourishment from the 
product of this destruction. In a diseased seed-bed these 
pr^ocesses are going on so continuously that the fungus 
soon produces many thousands of the motile Sj3ores which 
are able to attack new seedlings, thus accelerating the 
progress of the disease. In addition, as we have already 
seen, even the threads of the fungus grow across the soil 
between the seedlings and directly produce new infection. 

It is well known that "Damping off will recur with 
even greater virulence in seed-beds which show^ed the 
disease the previous season. This is explained by the 
existence of yet another chapter in the life history of 
Pythium, by which resting spores are formed that can 
pass the winter m the soil. At a late stage in the decay 
of diseased seedlings many threads of the fungus give 
rise to the resting spores. These possess the power of 
lying dormant over a long period, and in this resemble 
the seeds of higher plants, though they only consist of a 
single thick-walled cell. AVhen, however, the temperature 
is favourable and the conditions are sufficiently moist, the 
thick coat bursts and a fungal filament grows out which 
soon attacks any seedling that may be near, producing 
spore-cases and motile spores as before. The resting 
spores are produced in myriads m a diseased seed-bed ; 
in fact, on one occasion, I estimated the presence of upwards 
of half a million in a single diseased seedling observed 
under the microscope. As the seedlings rot these spores 
all find their way into the soil where they spend the 
winter. 



64 



In considering methods for preventing this disease it 
IS necessary to make use of the facts which are known of 
the means of spreading of the fungus causing the disease. 
We have seen that once a seed-bed is infected the fungus 
rapidly spreads, first by an abundant growth of filaments 
over the soil from seedling to seedling, and secondly by 
means of spores which swim in the water in the soil. Any 
treatment to be successful must check the vegetative 
growth of fungus in the soil and also the production of 
spores. It has been shown that the fungus only grows m 
this way in warm, very moist, badly ventilated seed-beds. 
Moreover, the eftect of excessive moisture and lack of 
sufficient air is two-fold. Xot only do such conditions 
favour the growth of Pythiiiiii^ but these verv conditions 
tend "to produce weak drawn seedlings which of course 
are more susceptible to attack. Very careful attention to 
cultural conditions will therefore make it possible to ward 
off" and. 111 most cases, entirely prevent the disease. The 
seedlings should never be overcrowded, and they should 
be given as much air and as little water as possible. If 
through inattention to these conditions a few of the seed- 
lings m a bed should show signs of disease they should 
be immediately removed and iDurned, and more air and 
less water given to those that remain. In any treatment 
it is also necessar;\' to prevent the infection of the seed-bed 
at the beginning of the season. This infection invariably 
arises from the presence of resting spores in the soil. 
Naturally, then, soil known to have previously produced 
the disease should not be used, but since the resting spores 
are present m most garden soils it is wisest to endeavour 
to get rid of them. They can be killed by partially 
sterilising the soil by heating with steam or with boiling- 
water. It has been shown in an earlier lecture that the 
partial sterilisation of soil is valuable for other reasons. 
By adopting the practice it is possible to eliminate the 
resting cells note only of PytJiiinw but also of other 
harmful organisms. 



Chapter 9. 



DISEASES OF ROOTS AND TUBERS. 

Fingers-and'Toes of Turnips, Cabbages, etc. Habits and life 
history of the fungus. Remedial treatment. "'Liming of the 
soil, etc. Similar diseases, ^-g-y Black Scab or Wart Disease of 
Potatoes, ''Spongy Scab,'' etc. 



The disease commonly known as Fingers-and-Toes, 
•or Club-Root disease, attacks all cruciferous crops, includ- 
ing Cabbages, Cauliflowers, Brussels Sprouts, Kohl Rabi, 
Broccoli, Turnips and Sw^edes. It is produced by a fungus 
known as Plasmocliofhora Brassicae^ and causes consider- 
able damage both in the gardens and fields in many dis- 
tricts of England ; its ravages are even greater on the 
Continent, and it is also abundant in America. 

The general appearance of the disease is well known. 
Plants, w^hich are attacked, show irregular warty swellings 
on the root, and in bad cases the whole root may simply 
consist of a mass of these excrescences. The disease can 
often be recognised m its earliest stages on pulling up 
young cabbages for transplanting. If such seedlings are 
planted they prove practically useless, for the disease 
simply increases at the expense of the roots, any leaves 
•developed being feeble and unable to provide for healthy 
growth. 



65 



66 



Seedling cabbages^ which show by the nodular swell- 
ings that they are attacked by Cl'ub-Root, are often 
exposed for sale. Of course it is inadvisable to plant 
such seedlings, for not only is it extremely unlikely that 
they will produce healthy plants, but when they decay in 
the ground they will infect the soil. If any plants of the 
cabbage family are grow m the same soil within two or 
three years of this infection, thev will almost certainly 
suffer from the disease to an even greater extent. 

The disease with which we are dealing should not l^e 
confused with the gall-like swellings on the roots of 
cabbages and turnips, sometimes caused by the larva of a 
beetle that lives within the cavitv of the swollen tissue. 
Eelworms, as well as certain bacteria, are also able to 
produce small swellings on the roots of many plants, in- 
cluding Crucifers. It is a little unfortunate that frequently 
any swellings on roots of cruciferous plants are spoken of 
as Club-Root without reference to the causal orgarfism 
Since, however, in the vast majority of such cases the slime 
fungus, Plasviodiofhora Brassicae, is the cause, I am re- 
stricting the name Club-Root or Fingers-and-Toes to 
the disease of which it is the cause. Whilst the life history 
of Pythinni and of the other fungi mentioned in the last 
chapter is typical the Club-Root fungus differs somewhat 
m life story and mode of nutrition. 

When one of the diseased roots, say of turnip, is cut 
across and examined under a microscope it is found that 
the tissues of the root are altogether abnormal, many of 
the cells being strangely altered. In the health^' root of 
the turnip we can distinguish an outer band of softer 
tissues surrounding a central core containing a certain 
number of woody elements arranged like the spokes of a 
wheel. Between the last-named are broad wedges of 
softer cells which are packed with the reserve food material 
that is stored in the root of the turnip m the form of 
sugar. The root increases in thickness by the growth and 
division of a layer of cells near the outer part of the 
central core. When attacked by the fungus of which we 
are speaking, the whole machinery for the growth of the 
root is, as it were, thrown out of gear; cells which normally 
would produce woody tissue simD^^' give rise to giant thin- 
walled cells, and the result is an excessive production of 
thin-walled tissues. This also occurs in the position of 
the tissues which normally serve for the conveyance and 



67 



Storage of the food materials manufactured in the leaves. 
The reserve supplies are thus tapped by the fungus, with 
the result that the quantity of sugar stored by the grow- 
ing root rapidly diminishes. In such instances many of 
the cells of the root are enormously enlarged and differs 
as regards their contents from those of a healthy root. 

The healthy cells are lined by the colourless, jelly-like, 
living protoplasm with its nucleus and contain cell-sap rich 
in sugar. Diseased cells differ from these in several re- 
spects. They are generally much larger; protoplasm is 
present as before, but it looks frothy and in it can be seen 
granular masses of other slimy substance which is the 
protoplasm of the fungus slowly absorbing that of its 
victim. It is a remarkable fact that the early effect of the 
parasite upon the cell is to stimulate it to enlarge and 
even to divide, and thus obtain more food material from 
the adjoining cells, and this is ultimately used by the 
unbidden guest. Gradually the protoplasm of the fungus 
increases in size until the whole of the protoplasm of the 
cell disappears. Then the fungus undergoes certain 
changes and soon the cell is seen to be filled with a large 
number of tiny round bodies. These are the spores of the 
fungus, and as they only measure one fifteen thousandth 
of an inch in diameter they can only be seen when very 
highly magnified. A single diseased cell w^ill contain at 
least 100,000 of these spores, and since the diseased part of 
a turnip, for example, contains many thousands of such 
cells it is. easy to see that in one such root millions of the 
spores of the fung-us are produced. 

If a very small piece of a diseased root is broken up in 
water myriads of the spores are liberated from the cells. 
After a few^ hours very remarkable changes can be observed 
to take place in these. Each tinv spore swells somewhat 
and then bursts producing a small hole in one side of the 
colourless membrane. Gradually the living protoplasm 
within squeezes its way through the aperture thus formed. 
As soon as it is free it begins to wriggle and move about 
as a minute speck of living protoplasm. The protoplasm 
at one end is drawn out into a fine hair, and the 
lashing of the w^ater by this hair causes the movement of 
the minute organism. After a time the movements of 
these specks of protoplasm become more sluggish and soon 
they only creep about by slow movements, first pushing 
one part of the protoplasm forward and dragging the 



68 



rest after it. While it is unknown how long- they can 
move about and hve in the soil, they can certainly do so 
for many days. 

In the field and garden the spores of this fungus are 
liberated into the soil by the decay of the diseased tissues 
of the infected roots. If diseased roots are left in the 
ground for any length of time the decay takes place 
rapidly and is generally accompanied by an offensive 
odour. The spores pass into the soil and there remain 
until conditions are favourable for their germination. If 
liberated during the summer or early autumn they pro- 
bably germinate at once, but if later it is likeh' that they 
remain m the spore condition over the winter and 
germinate m the warmer spring days. The smallest 
quantity of water is sufficient to allow the minute specks 
of protoplasm which are liberated, to swim about. If 
young cabbages, turnips, etc., are grown in the infected 
ground the organism soon gains entrance to the younger 
roots and sets up the disturbances described above. When- 
ever seeds of these cruciferous plants are sown in soil 
known to contain the spores of this fungus the swellings 
typical of the disease appear on the young roots in a few 
weeks. 

We have seen that this organism differs from a fungus 
like Fythiinii in several important respects. At no stage 
m its life history does it possess filaments as do the vast 
majority of fungi. It passes through the vegetative stage 
of its life as a naked speck of protoplasm living and grow- 
ing inside the protoplasm of the cells of a root. Unlike 
Pythiuvi it does not kill the cells of its host plant out- 
right, but rather stimulates them to enlarge, divide and 
draw food supplies to them which it then utilises. Like 
Pyihiiivi, however, it rests in the form of spores in the 
decaying roots and in the soil; it also passes part of its 
life as a naked free-swimming speck of protoplasm, but 
in order to complete its life cycle it must enter the living 
cells of the root of a cruciferous plant. 

In considering methods of preventing the attack of this 
organism it is necessary, as in all such cases of plant 
disease, to bear in mind the habits and life history- of the 
parasite. As has already been shown, a single diseased 
turnip or cabbage, if left to rot will literate many 
millions of spores into the soil. Some of these doubtl^s 
die, but many remain as a source of infection for future 



6g 



crops. Most important therefore, of all the means of com- 
batting this disease are the measures taken to prevent the 
spores of the fungus reaching and infecting the soil. It 
would seem to be obvious to anyone who has observed the 
damage this disease can cause, that the greatest care ought 
to be taken to collect and destroy by burning all diseased 
roots. Again and again this disease occurs both in fields 
and gardens, and in almost every case diseased roots can 
be seen left lying about, to rot and infect the land. Quite 
recently the writer watched a farmer carting the least 
diseased portion of a crop of swedes, scarcely a root of 
which had escaped attack. The most badlv diseased 
plants, already putrid and rotten, were being left in heaps 
to rot on the field, and then doubtless would be ploughed 
into the soil. In such cases not only does the soil of the 
field or part of the garden where diseased plants were 
grown become infected, but the fungus is carried on the 
boots of workers, on tools, or if, as so frequently happens, 
diseased roots are throvv^n on to the rubbish heap to rot, 
the disease ultimately gets spread over the whole field or 
garden. Diseased turnips are fed to animals, and though 
there is no direct evidence, it is quite possible that the 
spores pass uninjured through the bodies of the animals, 
and return to the soil in farmyard manure. The greatest 
care ought to be exercised to destroy all diseased material. 
Burning is the safest plan or, failins; that, it should be 
gathered into a heap and thoroughly mixed with quick- 
lime. 

The Club-Root disease has been noticed to be par- 
ticularly abundant on soils which are badly drained, at all 
sour, or deficient in lime; the disease is practically 
unknown in chalky or limestone soils. As direct methods 
of treatment therefore, the drainage of the soil should be 
improved, and a most drastic system of " liming adopted. 
Wherever the disease has been prevalent it is best to treat 
the ground immediately with freshly slacked quicklime at 
the rate of h to i cwt. per square rod, spreading it evenly 
over the ground and digging it in. Freshly prepared 
quicklim.e is much better for this purpose than ground 
lime, which varies considerably in the amount of active 
quicklime which it contains. Gas lime, though valuable 
as an insecticide, is a poor substitute for quicklime, since 
it contains a much smaller proportion of active lime than 
either of the other forms. 



70 



The use of certain acid artificial manures and, indeed, 
also of ordinary stable manure in excess, tends to make the 
soil acid, and this favours the growth of the Fingers-and- 
Toes fungus. In infected soil, therefore, Basic Slag, or 
other alkaline artificials, might be advantageously used 
m addition to treatment with lime. 

Needless to say, even after such treatment as outlined 
above, ground known to be infected should not be planted 
with cruciferous crops for at least three years. Other 
vegetables, including potatoes, can of course be grown 
without any danger, since they are not attacked by the 
fungus. The soil of beds m which seedling cabbages 
are raised should be partiallv sterilised as for the Damp- 
ing-off Disease, and also should contain a fair sprinkling 
of lime. 

By strict care m disposing of rubbish, by improving 
the drainage of the soil, and by regularly dressing the 
ground with quicklime, it should be possible to do some- 
thing towards eradicating this pest. A recent visit to some 
allotments within the ^vlanchester area aft^orded evidence 
of this possibility. Some of the plots had not grown a 
healthy cabbage for two or three years, while adjoining 
plots never show a sign of the disease. The infected 
plots were badly drained, the soil was sour, liming had 
been tried, but in too small quantities. On the other hand 
the adjoining plots had been thoroughly and regularly 
limed, were well drained, not soured by over manuring, 
and they had therefore always borne healthy crops. It is 
needless to add that on the infected plots diseased 
cabbages had been pulled up and the roots simply thrown 
aside to rot and prove a further source of infection. It 
might be well if allotment societies had some stringent 
rules for dealing with the spread of such diseases z'ic7 the 
rubbish heap. 

Brief reference must now be made to some diseases 
of the potato tuber that present certain features in 
common with the Fingers-and-Toes disease. The 
AVart Disease, or Black Scab of Potatoes, is caused by 
Synchytriinn enclobioticuni. It is notoriously prevalent in 
the districts round large towns, and the restrictions of the 
Board of Agriculture have rendered its symptoms well 
known. In the early stages small warty swellings appear 
in the eyes or buds of the potato, or the^' may even 
occur on the stem near the ground level. These warts 



71 



rapidly increase in size, and several often run together 
till the potato becomes a mass of excrescences. Plants 
which are attacked often grow larger and bear green 
leaves for a longer time than healthy plants. 

The fungus causing the disease, like the Plasmodio- 
phora, lives in the cells of the potato buds as minute 
specks of naked protoplasm. In this case the presence of 
the fungus causes the invaded cells to enlarge, but renders 
them incapable of further division. On the other hand 
the healthy cells around are stimulated to such active 
growth and division that the warts are soon produced. 
Infection always occurs by motile spores from the soil, 
which can only penetrate the healthy cells of a potato 
in the young condition. Later the potato forms a skin 
of corky cells through which the fungus cannot 
penetrate. After growing at the expense of the 
invaded cells the fungus ultimately occupies the whole 
of the cell cavity, and then taking on a thick, very re- 
sistant wall, forms a resting spore. With the decay of 
the warty tubers these resting spores find their way 
into the soil and may remain there as a source of infection 
for many years. When they germinate after an 
exceptionally long period of rest the thick wall bursts and 
liberates large numbers of actively moving spores, each 
possessed of a single whip of protoplasm to propel it. 
These are the spores which infect new potato tubers. So 
far no method of successfully treating this disease by 
adding chemicals to the soil has been devised, but certain 
varieties of potato are much less susceptible to the disease 
than others, and it is advisable to grow^ these if the 
disease is present. Indeed, it is now compulsory for every 
person growing, potatoes to do this and to follow^ certain 
other stringent regulations in areas where the Wart 
Disease is prevalent.''^ 

In addition to the Wart Disease there are several other 
scab diseases of potato tubers which, owing to a certain 
degree of similarity, may at times be confused w^th the 
Wart Disease. The Black Speck, or Violet Rhizoctonia 
Disease, caused by Rhizoctonia Violacea^ can be detected 
by the minute size of the blackish warts which can be 

^ See Board of Agricuhure leaflet 105 and Wart Disease 
Order, 1Q14, to be obtained free from 4, Whitehall Place. 
London, S.W. 



72 



rubbed off the surface of the tuber without causing any 
evident injury. The spongy or powdery scab, caused by 
Sfongosfora solani^ a fungus akin to the Wart Disease 
organism, is easily recognised by the light brown colour 
of the rather powdery warts, or cankers, it produces. 
BrowTi Scab, or Ordinary Scab, of the tuber also shows as 
light brown warts, but usually evenly distributed over the 
surface of the potato. It may be due to a variety of 
causes, in which mechanical irritation by gritty 
particles of soil and infection by definite parasitic 
fungi probably play an important part. None of these 
diseases, however, are so destructive as the Wart Disease; 
further particulars of them may be obtained from the 
leaflets issued by the Board of Agriculture. t It is quite 
certain that these scab-like injuries are caused by different 
fungi, but it is not surprising that the resulting diseases 
present certain superficial similarities. In each case the 
presence of a parasitic fungus irritates the cells of a 
potato, causing them to actively divide and thus give rise 
to the warts. Even in the Fingers-and-Toes disease 
the result is similar, and in all the cases with which we 
have dealt the attack arises from the presence of the 
organism m the soil. By, therefore, adopting precautions 
to prevent the infection of the soil, and by appropriate 
treatment similar to that advised for the Club-Root 
disease, such diseases can, in some measure, be controlled. 

t See Leaflets 171^ 137 and 232. 



Chapter 10. 



A DISEASE OF LEAVES, SHOOTS, AND TUBERS. 

The Late-Blight disease of potatoes. Symptoms and means of 
spreading. Diseased tubers and the wintering of the fungus. 
Treatment of the disease, spraying and the use of resistant 
varieties. 

The Late-Blight of potatoes caused by Phyto-phthora 
infestans is so common and wide-spread that it is generally- 
spoken of as the Potato Disease. Historically, it is cer- 
tainly the oldest of the potato diseases, and it is probably 
responsible for more loss to potato growers than all the 
other diseases of the potato put together. The malady 
fir-st made its appearance in Europe about 1840, and the 
Irish Potato Famine of 1845 was the result of a very severe 
attack of Phytofhthora. Whilst its ravages are seldom 
so severe nowadays as they wxre in that year, the disease 
is never altogether absent In most seasons it makes a 
hrst appearance about the middle of July, and from that 
date to the end of the potato season it is often difficult 
to find a field of potatoes wholly free from attack. 
Farmers and growers generally, however, do not seem to 
be seriously concerned so long as the tubers are not badly 
diseased. It is an important fact, however, that wherever 



73 



74 



the disease is present at all, there is a risk of a proportion 
of the tubers becoming; diseased; further, the attack of 
the fungus on the leaves invariably results in a diminished 
crop. The first outbreak of the disease in the season is 
usually dependent upon the vreather conditions. In this 
country a few days of close, moist weather about the 
middle of July are often followed by an outbreak of the 
disease, and, if such weather persists, an epidemic fre- 
quently results. 

The disease is easily detected by the characteristic spots 
on the leaves. These often appear near the margin or tip 
of the leaf and they usually present a dull, water-soaked 
appearance. As the disease progresses the infected spots 
increase in size, become darker in colour, and may rapidly 
involve the whole leaf -blade, extending from the latter 
to the petiole and even to the stem. Ultimately the leaves 
hang limp and the whole plant becomes moist and 
blackened. Such diseased plants emit a peculiar, offensive 
odour which is very characteristic of the disease. If the 
attack is a severe one, it is generally found on lifting the 
tubers that they also are diseased. If in the earliest stages 
of the disease, the dark spots on the leaves are closely 
examined on the under surface, a delicate, white, mildew- 
like growth will be evident round the margin of the dis- 
coloured spot. This growth follows the advancing 
margin as the diseased area increases in size. Microscopic 
examination shows the mildew appearance to be produced 
by filaments of the fungus Phytofhthora, which oroject 
from the lower surface of the leaf. The growth as a rule 
has a somewhat powdery appearance produced by large 
numbers of spore-like bodies, which readily fall free from 
the filaments bearing them. Thin sections through a 
diseased spot show that at this stage the threads of fungus 
run between and within the cells of the leaf. Alany of 
these cells are shrivelled and dead, and the contents 
having turned brown, they contrast strangely with the 
taut living cells lined wdth colourless protoplasm and 
green granules of chlorophyll. 

In many places branches from the filaments of the 
fungus pass out through the air pores or stomata, which 
are especially abundant on the lower surface of the leaf. 
These aerial filaments become branched and bear small 
lemon-shaped bodies, called conidia, on the tips of the 
branches. The production of such conidia enables the 



75 

fungus to spread with amazing- rapidity ; when mature 
they are easily detached, and being extremely small the 
slightest current of air carries them in large numbers to 
the leaves of adjoining plants. The conidia are only 
produced when the air is moist and sultry. Alighting 
upon the healthy leaf of the potato, under such atmospheric 
conditions they germinate in one of tw^o ways. Either, 
each conidium behaves as a spore-case and liberates motile 
spores, or it germinates by sending out a hne filament. 
By the former method it opens at the tip and liberates 
about eight small motile spores w^hich swim about vigor- 
ously in the thin film of water on the surface of the leaf. 
As in the case of Fythium, the fungus causing " Dampmg- 
off " of seedlings, these motile spores are each furnished 
with two hairs of protoplasm, by means of which they are 
enabled to move m the water. After a short period the 
motile spores come to rest, withdraw the minute hairs and 
give rise to a short filament which either grows through 
one of the stomata into the leaf, or dissolves a way 
through the outer skin of the leaf, and so starts a disease 
spot. In other cases the conidium simply sends out a fine 
tube and produces an infection by the same methods as 
do the motile spores. After infection the fungus, feeding 
on the cells of the host, grows rapidly, in the form of 
branched threads, within the tissues of the leaf. The 
conidia are extremely small, being less than one- 
thousandth of an inch in length. Enormous numbers are 
produced from a single diseased area of a leaf, and, 
further, under suitable conditions the fungus grows and 
reproduces itself very rapidly. It is therefore easy to see 
why, in w^eather conditions favourable to its growth, the 
fungus spreads in a few days over whole fields of potatoes. 

In the earlier chapters it has been shown that a leaf, 
such as that of the potato, is really a complicated machine 
for the manufacture of food material. Many of the cells 
contain chlorophyll, the green colouring matter w^hich, in 
the presence of sunlight, utilises carbon-di-oxide obtained 
from the air to produce organic chemical compounds like 
sugar and starch. The complex organic substances 
elaborated in the leaves are passed on to the stem and 
ultimately stored m the potato tubers in the form of starch. 
Thus it is obvious that by injuring the leaves, the potato 
disease reduces the weight of tubers obtained from a given 
plant, and the earlier the attack occurs in the season, the 



76 



more serious is this effect on the crop. In most years 
potato plants which are attacked by Late-Blight, die down 
much earlier than they would normally. 

When a field has been badly attacked by Late-Blight 
the tubers almost invariably become infected. This either 
takes place by spores being washed down into the soil 
and directly infecting tubers near the surface, or by the 
growth of the fungus down the tissues of the dying stalks 
into the tubers. Potato tubers infected by this disease 
can readily be recognised by the purplish discoloration 
and rather sunken appearance of the skin in diseased 
parts. These features are caused by the brown colour 
of the cells immediately under the skin. Living filaments 
of the fungus infest the discoloured cells, and if such 
tubers are stored the fungus slowdy invades healthy cells^ 
often producing the so-called dry rot of the tuber. In 
addition to this, the presence of the fungus in the potatoes 
renders them extremely susceptible to the attacks of 
secondary rotting fungi and also of bacteria. In some 
such cases the potato shows the characters of the winter 
rot disease caused by Fitsariiwi solani. The tuber 
gradually shrivels and at a late stage small white tufts 
of that fungus appear over the sunken parts. In other 
cases the potato becomes changed into a moist, putrid 
mass infested by bacteria and mites. These diseases are 
extremely likely to appear in stored tubers that are to 
any degree infected by Phyto-phthora. Much, how^ever, 
can be done to prevent healthy tubers developing these 
rots by careful attention to the method of storage. It 
is well known that the favourable conditions for the 
growth of most fungi are a moist, warm atmosphere 
and absence of light. If, therefore, the storage clamp is 
not carefully made, it is possible that very favourable 
conditions for the growth of fungi will be provided. If 
the tubers are stored moist, or if they are too closely 
covered, and if no provision is made for adequate access 
of air to all parts of the "pie," the temperature wall rise 
and harmful fungi will become rampant. To avoid these 
dangers then, potatoes should be stored in a dry, well- 
ventilated shed ; or, failing that, the " pie " or clamp should 
be in a dry situation and well ventilated. By such pre- 
cautions the conditions are rendered unfavourable for the 
growth of rotting organisms; and even though a few of 
the tubers are diseased the trouble is then unlikely to 



77 



spread to the healthy tubers. Too frequently, however, 
it is found on opening a clamp that the whole store is a 
putrifying mass, smiply owing to neglect of the essential 
facts of ventilation and dryness. 

We have seen that the Phytophthora spreads through- 
out the summer months with amazing rapidity by air- 
borne conidia. If, however, these are to produce infection 
they must germinate immediately, for being only provided 
with a thin wall they cannot resist drying up or frost. 
The}', therefore, never serve for carrying over the fungus 
from one season to the next. 

1 he question then arises as to whether this fungus, like 
PytJitiiin, produces resting spores that are able to survive 
the winter in the soil. The most diligent search of many 
investigators has failed to show that such resistant resting 
spores are ever produced in Xature. On the other hand, 
Dr. Pethybridge, in Dublin recently, has found it possible 
to obtain resting spores of this fungus by growing it 
artificially on a medium consisting of cooked Quaker 
Oats stiffened with a gelatinous substance called agar. 
The fungus forms a dense, white growth on the surface of 
this, and after some months produces the resting spores 
within the medium. These are quite characteristic resting 
spores and possess a thick resistant wall. Interesting as 



scientific point of view^, the fact that their presence has 
never been demonstrated in Xature, renders it necessary 
to consider other means by which the fungus may survive 
the winter. It has been repeatedly shown that diseased 
tubers kept over winter m the open may give rise to a 
growth of Piiytofhthora bearing spores in the warm, moist 
days of early summer. During the colder months fila- 
ments of the fungus m diseased tubers grow very slowly 
indeed, especially if the tubers are kept dry. If, how- 
ever, such tubers are kept warm and moist the fungus 
rapidly extends through the whole tuber and even pro- 
duces a network of threads bearing powderv conidia on 
the outside. Such conidia, carried in the air, infect leaves 
of potato plants m the vicinity, and in this way start an 
epidemic. It has frequently been noticed that portions 
of a potato field, near to old potato pits or refuse heaps, 
have been the starting points for the disease. This mode 
of initial infection is, however, scarcely enough to account 
for the very wide-spread freouency of the disease. 



these 




resting spores are from the 



78 



Recent investigations, especially m Ireland and 
America, have done much to make it clear how most 
epidemics of Late-Blight originate. We have seen 
that the potato tubers very frequently become infected by 
the fungus. If badly diseased tubers are used for seed 
it has been found that they either wholly decay m the 
ground or occasionally send up a few perfectly healthy 
shoots. Recently, however, very careful field experiments 
have shown that if only slightly diseased tubers are 
planted^ a much larger proportion of them send up shoots, 
and under certain weather conditions some of these become 
diseased. It has also been proved that the disease arises 
m such young shoots by the growth into them of the 
fungus from the slightly diseased tuber. Dr. Melhus has 
confirmed De Bary's earlier observations that such 
diseased shoots occasionally reach the surface where 
conidia of the fungus are produced. These conidia are 
then carried to the leaves of adjoining plants, which very 
soon show typical disease spots. Upon these, more conidia 
arise and so starting with a single diseased shoot as a 
centre the disease spreads over the whole field in a few 
warm moist days. 

In taking measures to prevent the Late-Blight disease, 
it IS necessary, as in other plant diseases caused bv fungi, 
to consider the means by which the fungus spreads during 
the season, and also the manner in which it is carried 
over to the following year. It has been shown that this 
disease spreads very rapidly throughout the summer, by 
means of air-borne conidia that infect the leaves on which 
they alight. If the germination of conidia can be pre- 
vented then the spread of the disease will be controlled. 
This may be accomplished by spraying with a fungicide, 
that is, a poisonous substance which is harmful to germi- 
nating spores. It has been found by experience that one 
of the most powerful mixtures for preventing infection by 
filaments from germinating spores of fungi is Bordeaux 
mixture. This consists of a solution of Blue stone, that 
is copper sulphate, to which slaked lime or lime water is 
added. The lime is added to prevent the copper sulphate 
injuring the leaves of the plants treated, and also to make 
the mixture form a fine film on the surface of the leaves. 

Bordeaux mixture should always be freshlv prepared^ 
and the home-made article is much better than any adver- 
tised preparations. It is a fairly simple matter to make 



79 



the mixture, and full directions for its preparation and 
use are given by the Board of Agriculture in Leaflet 23. 
Once the disease is evident in the field it is too late to 
attempt to control it by spraying, but the first application 
should be made some time before the first outbreak of 
disease is expected, i.e., about the end of June or beginning 
of July in most districts. The mixture should be well 
stirred before use and both surfaces of the leaves of the 
plants should be thoroughly sprayed. Three or four 
sprayings at intervals of a fortnight or ten days should 
generally be sufficient to prevent any serious epidemic. 
The cost of such sprayings is relatively small, working 
out m normal times to about 25 /- an acre for three applica- 
tions [i.e., 2d. per sauare rod}. Apart even from the 
prevention of Late-Blight spraying has been proved bene- 
cial to the plant ; for sprayed plants usually retain their 
leaves at least a fortnight longer than unsprayed. In 
certain districts of America where spraying is regularly 
adoDted the increased profit is usually from to £^ per 
acre. 

The subject of the resistance of different varieties of 
potatoes to this disease has received the attention of ex- 
perimental growers for many years. Darwin, himself was 
for some years interested in the matter, and as a result, 
various species of wild potatoes growing in South America 
were experimented with and used for crossing. At the 
present time, therefore, something can be done to avoid 
the disastrous effects of the Late-Blight disease by grow- 
ing varieties known to be resistant to this disease. It must 
be said, however, that though a variety is highly resistant 
in one locality it may be equally susceptible in another 
district where the conditions of environment differ. It is 
also notorious that disease-proof varieties lose their resist- 
ance after a few years, either because the plant, in time, 
deviates from the original type, or because the fungus 
becomes slightly modified so that it is able to break down 
the resistance, ^s'eedless to say, a variety which is resistant 
to Late-Blight disease will not necessarily be resistant 
to the Wart disease and other diseases of the potato. 
Since the Late-Blight disease usually affects the crop most 
seriously towards the end of the season, it follows that 
early varieties do not suffer from the disease to the same 
ex-tent as the main-crop and later varieties. Bv, there- 
fore, selecting varieties proved to be resistant in recent 



8o 



years in the given district, and by choosing early varieties 
it is possible to avoid this disease to some extent. 

It IS even more important, however, to endeavour to 
prevent the f±rst outbreak of the disease in a given area. 
All possible sources of infection such as diseased tubers 
and stalks should be carefully collected and burned at 
the end of the season. Since the disease is propagated 
from year to year in slightly diseased tubers, seed potatoes 
should only be used from a crop which never showed 
any sign of the disease even on the leaves. 

We have seen that tubers become mfected either, by 
the growth of fungus down the stalks, or by spores being 
washed into the soil. If, therefore, the attack occurs late 
in the season all haulms should be removed and destroyed 
before the tubers are lifted; and infection from the latter 
source may be prevented, to some extent, by seeing that 
the tubers near the surface are well covered with soil. 
This should be done as soon as possible after the haulm 
has died down, for apart from the risks of the Phyto-ph- 
thora disease, tubers left too long in the ground are 
more liable to be attacked later by rots in the store. 
The tubers should be stored under dry conditions in such 
a way that air has free access to them, and diseased 
haulms should never be used for covering the '^pie" or 
clamp. 

If potato plants are sprayed three or four times at 
fortnightly intervals, beginning when they are about six 
inches high, it should usually be possible to prevent any 
serious epidemic in a given field or garden. Much can 
also be done to prevent infection by the immediate destruc- 
tion of diseased tubers and haulms, and by only using 
seed potatoes from a perfectly healthy crop. By such 
precautionary measures, in addition to the use of varieties, 
known by experience to be resistant in the district, it 
should be possible to reduce the effects of this disease to 
a minimum. 



Chapter 11. 



DISEASES OF LEAVES AND SHOOTS fcontdj. 

Black-Leg or Wilt Disease of Asters. Tomaio-leaf rust or 
mould Leaf-blotch of Cucumber, Mildews of Roses^ Goose- 
berries, etc. 

The Black-Leg or wilt disease of asters is extremely 
prevalent wherever asters are grown. It is caused by a 
species of Phytophthora, which differs in several respects 
from Phyto-plithora infestans, the cause of the Late-Blight 
disease of potatoes. The disease may manifest itself at 
any stage m the life of the aster, but the initial attack is 
alw^ays upon the seedlings. In severe cases the seedlings 
collapse, as in the Damping-off disease caused by 
Pythium, but more frequently they harbour the fungus 
without showing any outw^ard sign of disease at this stage. 
Such seedlings may wilt and collapse when transplanted, 
but many succeed in surviving even to the flowering stage 
without show^ing external signs of injury. In the latter 
cases the plants succumb quite suddenly, almost without 
warning. The leaves w^lt, hang limp and flaccid, and in 
a few days the whole plant shrivels and dies. Even, 
though affected plants show^ little direct sign of the disease 
until the wilt sets in, they are often much dwarfed in size 
and produce fewer flowering branches than healthy plants. 
The fact that this disease may be present in apparently 
healthy asters, which only wilt after they come into flow^er, 



8i 



82 



renders it all the more objectionable and difficult to com- 
bat. When wilted plants are pulled up, the parts of the 
stem a few inches above the level of the ground are found 
to be blackened and often decaying; these symptoms have 
given the disease its popular name. 

The parasitic fungus which causes the aster disease, 
unlike Phytophthora mfestans of the potato, never attacks 
the leaves directly but enters the plant from the soil 
through the root of the seedling. Once withm the young 
plant it may, either grow slowly not seriously interfering 
with the work of the vital parts"^of the seedling, or it may 
extend rapidly through these and cause immediate col- 
lapse. Alicroscopic examination of the blackened portion 
of the stem shows the distribution of the fungus m the 
tissues. The cells of the rind as well as the spaces betw^een 
them are occupied by filaments of the fungus. The 
former are not so rapidly killed by the fungus as are 
those of the potato plant by Phytophthora injestans. 
The ultimate collapse of the plant, however, is brought 
about b}' the extension of the fungus to the vascular 
cylinder of the stem, and the consequent reduction of the 
supply of water to the leaves. 

In moist weather the fungus gives rise to conidia on the 
diseased stem. Unlike those of Phytophthora injestans 
they are only produced under water, and do not become 
detached from the filaments bearing them. A few hours 
after their first appearance they burst at the apex liberat- 
ing about fourteen motile spores which, after swimming 
about for a short time, come to rest and are able to infect 
other seedlings. 

In studying this disease the writer has up to the present 
failed to find any resting spores of the Phytophthora in 
the tissues of diseased asters, and attempts to obtain such 
spores by artificial cultivation have been equally unsuc- 
cessful. Experience however has shown quite conclusively 
that the disease originates each season from the presence 
of the fungus in the soil, especially of the seed bed ; and 
the ease with which it mav be cultivated on dead organic 
substances suggests that it may be able to persist in a 
vegetative condition in the rich soil used for seed-beds. 
On the other hand, further research may prove the exist- 
ence of resting spores. 

It is often stated that this disease is caused by a species 
of Fiisariuvi, which is frequently found on the decaying 



83 



tissues of diseased asters. The writer," however, has 
proved that a species o£ F Jiyt ophthora is the primary cause 
of the Black-Le^ disease and that Fusariinn only appears 
later as a saprophyte living upon tissues previously killed 
by the other fungus. The Fhytofhthora is always present 
m the diseased asters even m the earliest stages ; it has 
been isolated and grown separately from all other organ- 
isms, and the disease has been produced artificially from 
such growths. Other organisms found on diseased asters, 
including Fii sarin, vi, are unable to infect and produce wilt- 
ing m healthy plants; such saprophytes onl}' succeed after 
the tissues have been killed by the FJiyioplitJiora. 

The insidious nature of this disease renders it extremely 
difficult to deal with, for it is usually almost impossible 
to detect diseased plants until the wilting actually sets in. 
Whilst sufficient scientific trials of remedial measures have 
not yet been made to warrant promise of complete success 
111 all cases still some precautions m.ay be indicated. 
The soil of the seed-bed should be partially sterilised 
by steam or hot water and asters should not be planted in 
ground which produced diseased plants the pjrevious sea- 
son. All diseased material should be removed and 
burned and the infested soil thoroughly drenched with 
Formalin \i pint per lo galls, water' and coyered with 
sacking for a few days. ]\Iany growers raise aster seed- 
lings on hot-beds of stable manure, but these conditions 
should be avoided since they are much more favourable 
to the fungus than when alkaline artificial manures are 
used. 

As a first example of the common diseases of leaves, 
the Xomato-leaf rust or mould caused by Cladosforhivi 
fidvimi may next be considered. This is more strictly a 
disease of the leaves than is the Late-Blight of potato, 
but even in this case the whole tomato plant suffers 
because of the attack on the leaves. The disease, which 
has been known in this country for over a quarter of a 
century, first appears on the leaves in the form of small, 
yellowish spots. These gradually increase in size and 
often run together, and the under surface of the leaf 
in the diseased areas becomes covered with a rust}/, velvety 
growth. This is the reproductive part of the fungus pro- 

Annals of Apphed Biology." Vol. II., Nos. 2 and 3. July, 
1Q15, pp. T25-137. 



84 



ducing the disease. The vegetative filaments ramifying 
in the tissues of the leaf give rise to branches which pass 
out through the stomata and stand erect from the surface 
of the leaf. Food material is absorbed from the host 
plant by the vegetative fiJaments and is passed on to the 
reproductive branches. These produce large numbers of 
small two-celled spores, which being readily detached are 
disseminated by air currents. Under suitable conditions 
they germinate immediately on the surface of tomato 
leaves; each of the cells of the spore may send out 
a filament, which growing through one of the stomata, 
into the interior of the leaf produces a new infection. 
The careless watering of slightly diseased plants may 
carry spores to healthy leaves and thus spread the disease 
which is highly infectious. 

The disease only occurs in this country on tomatoes 
grown under glass, and though the spores described above 
are the only kind known, it is certain that they are able to 
survive the winter in the greenhouse and give rise to infec- 
tion the following season. If the houses are kept suffi- 
ciently well ventilated the disease seldom assumes serious 
importance ; neglect of this precaution may, on the other 
hand, prove disastrous. It is possible to prevent any bad 
outbreak by regularly spraying with a Bordeaux mixture 
of half the usual strength, until the young fruit is begin- 
ning to set, when, owing to the poisonous character of 
Bordeaux mixture, Liver of Sulphur 'i oz. per 4 galls, of 
water) should be substituted. In order to prevent the 
disease recurring, all diseased leaves should be picked oft^ 
immediately and dropped into a vessel containing a 
solution of copper sulphate; at the end of the season 
remains of plants should be burned and the greenhouse 
disinfected in the manner to be described in connection 
wdth the Cucumber-leaf disease. 

Several other diseases of the tomato plan t,_^ which are 
readily distinguished from the Leaf-rust described above, 
m^ay be mentioned. The Sleepy disease caused by 
Fii'sarimii lycopersici. is a v\-ilt disease somewhat similar 
in symptoms to the Black-Leg of asters although caused 
by a very different fungus. llie Black-Stripe disease 
shows itself on the fruit and also sometimes on the stem 
as a dark, velvety growth of fungus. This disease, caused 
by Macros poriitm solam, should not be confused with the 
Bacteriosis of the tomato in which blackening of the parts 



85 



attacked is also produced. In Ihe latter case, BaciUus 
solajiacearnin is the cause. The Septoria disease of the 
leaves produced by Seftona lycoijersici is the only other 
malady likely to be confused with the leaf-rust, but m 
this case the spots are always small and concentric, and 
the spores are produced in minute black bodies scattered 
over the patches. 

The Leaf-Blotch of the Cucumber caused by Cercosfora 
jnelonis is another destructive disease of leaves. It was 
first described in this country by Dr. C. Cooke m 1896, 
and since then it has become so wide-spread that many 
horticulturists have been compelled to cease growing- 
cucumbers. Once the fungus appears in greenhouses it is 
extremely difficult to eradicate. The leaves are most often 
attacked, but the fungus frequently spreads to the fruit. 
An outbreak of the disease is usually first indicated by 
the appearance of pale, scattered spots on the leaves. 
These spots gradually increase m size, become brown, 
and the leaves are so rapidly killed that death of the 
plant may soon result. 

}>Iicroscopic examination shows that, m the region of 
the spots, the tissues of the leaf are occupied by filaments 
of the fungus, that the chlorophyll bodies are pale 
111 colour and many of the cells of the leaf are shrivelling 
and dying. From the fungus within the leaf stiff branched 
filaments grow out and stand more or less erect from the 
surface. These aerial threads are dark m colour, and 
bear numbers of large conidia which fall free as they 
mature. Each somewhat spindle-shaped conidium is 
divided into about seven or eight cells, and may germi- 
nate in a warm, moist atmosphere by sending out fila- 
mentous germ-tubes from any of the cells. The germ- 
tubes may then produce new infections by growing through 
the stomata into the healthy tissues of the leaf. This 
spreading by means of conidia that germinate imme- 
diately, takes place very rapidh* under favourable con- 
ditions. In addition to producing large conidia on the 
leaves the fungus is said to grow as a saprophyte on decay- 
ing leaves and damp soil producing myriads of smaller 
spores which also rapidly spread the disease. If the con- 
ditions are unfavourable to the germination of the spores, 
and especially at the end of the season, the large conidia 
persist alive as resting spores. Filaments of the fungus 
are also able to pass into a resting condition m the soil, 



86 



only to begin active growth with the production of spores, 
when the conditions are once more favourable. In this 
way the disease survives in greenhouses from one season 
to the next, and once a house is infected the disease is 
almost certain to recur year after year unless precautions 
are taken to thoroughly disinfect the soil and all parts of 
the house. 

The practice of disinfecting greenhouses with burning 
sulphur IS largely employed in some districts. Whilst 
this IS an excellent preventative of insect pests and 
of certain mildews, it is useless against the Leaf-Blotch 
of cucumbers. The writer recently established this with 
certainty. A large house which had been badly infected 
with the disease was cleared out and thoroughly disin- 
fected with burning sulphur. A few^ fragments of 
diseased leaves and fruits were then collected from the 
soil of this greenhouse and brought into the laboratory. 
Spores of the Cercosfora^ taken from this material, ger- 
minated in water in a few hours and cultures of the fungus 
were readily obtained on suitable media. From this 
experiment it is clear that the spores and resting filaments 
of Cercospora meloius remain uninjured in houses dis- 
infected by burning sulphur alone. Probably more certain 
results w^ould be obtained by spraying the house 
thoroughly, and also drenching the soil with Formalin 
(i pint per 20 galls, of water). The Board of Agriculture 
recommends the use of Jeyes fluid (i oz. per gall, of w^ater) 
for this purpose, but the wTiter has no experience of this 
as a disinfectant against parasitic fungi. Needless to say 
all diseased material should be destroyed by burning. It 
is possible to control this disease to some extent by spray- 
ing with Liver of Sulphur (^potassium sulphide) two ounces 
in three gallons of water, to which two ounces of soft soap 
is added to facilitate the sticking of the solution to the 
leaves, which should be thoroughly wetted by the spray. 
I'^he disease is only prevalent where cucumbers are forced 
under glass, but if the ventilation is carefully regulated 
much can be done to reduce the possibility of an epidemic. 
A most effective way of combating it is by growing 
disease-resisting varieties of the cucumber, of v/hich there 
are a number on the market. The most reliable of these 
have rather coarse, hard foliage, but unfortunately the 

^ Leaflet No. 76. 



8; 



fruit is not so highly valued as that of some of the more 
susceptible varieties. At the same time it ought to be 
quite possible to produce, by crossing, a variety which 
combines the qualities of disease-resistance and those most 
acceptable m the fruit. 

Many diseases of leaves belong to the class spoken of 
as mildews arid are caused by fungi belonging to the 
family ErysifkacecE. The Rose mildew, caused by 
Sfhaerotheca fannosa^ is one of the most familiar of these 
diseases and is typical of the class. The diseased leaves 
become covered wdth a white, pow^dery growth of the 
fungus, which causes them to curl up and che. The 
fungus grows mainly on the surface of the leaves, 
swollen branches from the filaments acting as sucker-like 
organs of attachment, while other branches penetrate the 
outer walls of the epidermal cells and swell out within the 
surface cells in the form of bladder-like haustoria. These 
absorb food material from the cells occupied as well as 
from the adjoining cells. The substances thus absorbed 
from the living leaf are passed on to the vegetative fila- 
ments of the fungus outside w^hich is thus enabled to grow 
and multiply. Erect threads arise from the creeping fila- 
ments on the surface and bear single chains of colourless, 
thin-walled, oval spores or conidia. Myriads of these are 
produced on the surface of mildewed leaves and give to 
the latter the characteristic powdery appearance. Being 
extremely light they are easily spread by the wind to 
healthy leaves and produce new infections throughout the 
summer. Later in the season, the production of conidia 
gradually gives place to another means of spreading. The 
fungus on the shrivelling leaves and also on the twigs 
assumes a brown colour and gives rise to minute dark 
bodies about the size of a pin head. This is the winter or 
resting stage of the fungus. Each of the minute dark 
bodies or perithecia is furnished w4th a thick wall made 
up of a number of closely interwoven fungal threads • and 
within this resistant coat a club-shaped spore-case {ascus) 
containing eight oval spores, gradually develops. When 
mature the perithecium splits across and the ascus is 
squeezed out. The latter then opens at the apex and the 
oval spores are forcibly ejected. These are able to infect 
leaves with the mildew and serve to start the disease afresh 
each spring. As has been stated above, many destructive 
mildews are caused by fungi having a life history closely 



88 



corresponding to that of the Rose mildew. The mildews 
of apple, chrysanthemums, peas, hops, strawberries, and 
gooseberries are all common m this country. The 
American Gooseberry mildew is the most destructive of 
these, and growers of gooseberries have to observe certain 
restrictions prescribed by the Government in regard to this 
disease. Full particulars of the symptoms and treatm.ent 
may be obtained from the Board of Agriculture.'^ 

Preventive measures against most of these mildews are 
similar. The main facts to consider are the means of 
spreading during the season and the method of carrying 
over from autumn to spring. Sulphur, or one of its com- 
pounds, is the mxost effective fungicide for use in combat- 
ting mildews. Plants may be dusted with flowers of 
sulphur or sprayed with liver of sulphur. Sprays, con- 
taining lime and sulphur, are now extensively used with 
success, especially in America, against the mildews of 
hops . and gooseberries. t The treatment with sulphur, or 
its compounds, however, is only effective against the 
summer stage of the fungus and other measures must be 
taken to destroy the resting fungus. The only satis- 
factory method of dealing with this is by the removal 
and immediate destruction of all branches and leaves 
bearing perithecia. Otherwise no amount of spraying will 
prevent the disease recurring year after year. 

^ Leaflet Xo. 105. 

t Eyre and Salmon have recently recommended the use of 
ammonium sulphide against American Gooseberry Mildew. 
•■'Journal Board Agric," Feb., igi6. 



Chapter 12. 



DISEASES OF LEAVES (contd.). 

Rusts. The Wheat Rust. Life Story of the fungus, relation to 
the Barberry, The Mint Rust and Hollyhock Rust as examples 
of other Rust diseases. 

The Rusts constitute a very important group of plant 
diseases, which includes some of the most widely dis- 
tributed and highly destructive of parasitic fungi. The 
grain-producing plants and grasses of our fields, the 
plants of our gardens and greenhouses, and even forest 
and fruit trees are attacked by members of the rust 
family of fungi. Of all plant diseases they are in many 
respects the most remarkable, as they are also the most 
difficult to combat. Whilst the nature and the life history 
of the fungi causing them have only been knovvn for a 
relatively short period, the blighting effects caused by out- 
breaks of these maladies are referred to by writers of the 
remotest antiquity. Throughout the centuries wheat rusts 
have been responsible for great damage wherever wheat 
has been grown, and at the present time the loss caused 
to the wheat crop alone throughout the world amounts to 
many millions of pounds annually. 



90 



Since the life story of the fungus causing the Black 
Kust of wheat {Piicczma graminis) is typical of man\' 
rusts, It will be described in some detail. The disease 
usually appears in the summer on the leaves and stalks of 
the ^growing wheat m the form of yellowish streaks, 
at first shining through the epidermis. The orange 
patches on the leaves consist of the developing uredo- 
spores of the fungus causing the disease. As this pro- 
gresses, the epidermis is ruptured and the bright orange 
uredospores are liberated as a fine powder. These spores 
bring about the rapid spread of the maladv from one 
plant to another throughout the summer months, so that in 
a short space of time whole fields of wheat may be rusted. 
As the disease pustules increase m number and size the 
leaves lose colour and become paler day by dav and a 
badly diseased field may thus give the appearance of 
premature ripening. 

With the advance of the season the streaks or the 
plants gradually change colour from orange almost to 
black, and although for a time some uredospores continue 
to arise m the pustules, mingled with them are now dark, 
brown spores of a different appearance. Finally, in the 
autumn, none but the dark spores are produced in the 
pustules. These spores are the teleutospores which serve 
for carrying the fungus over to the next season Before 
considering in detail the form and behaviour of the uredo- 
spores and teleutospores, reference must be made to an 
opinion which prevailed at least a hundred and fifty years 
ago with regard to the wheat rust. 

It was strongly held by farmers that the presence of 
bushes of the commion barberry near to wheat fields, bore 
some relation to outbreaks of rust, but no very definite 
reasons were given for this belief. The farmers of Massa- 
chusetts were so convinced of the connection between the 
barberry plant and the wheat rust, that a law was passed 
in 1755 compelling the destruction of all the barberry 
bushes. About a century ago Sir Joseph Banks suggested 
that a certain bright yellow fungus common on the bar- 
berry might be the same as that causing the rust of wheat. 
This fungus on the barberry, however, when examined 
microscopically was so unlike the fungus on wheat that 
for a long time the relationship was not understood. 

If a leaf of wheat is cut across through one of the 
yellow streaks and examined under the microscope, it is 



91 

found that the pustule is formed by the rupture of the 
epidermis of the leaf. Large mimbers of the orange 
uredospores arise under the skm and burst through to the 
outside. The torn edges of the epidermis thus act as a 
^boundary to the pustule. The filaments of the fungus 
ramify in the tissues of the leaf often sending special 
branches called haustoria into the living cells from which 
nutriment is thus absorbed. The reproductive branches 
of the fungus grow outwards and accumulate in rows 
under the skin of the leaf. The round, orange uredo- 
spores are produced on the tips of such branches,, and 
are liberated when the epidermis bursts under the pres- 
sure. Each uredospore is a single, oval cell about a 
thousandth of an inch in length, and readily falls 
free from the stalk bearing it. The wall of the spore is 
studded with minute w^arts and has four thin round pores 
near the middle part. The uredospores are able to ger- 
minate, immediately they are liberated, m a film of water 
or in damp air. Through the pores mentioned above two 
or three fine filaments grow out; one of these usually out- 
strips the rest and may becomie a long, branched, wa^ y 
filament. If in water alone, or indeed apart from a living 
leaf of the wheat plant, this filament is only able to survive 
until the small store of reserve food material in the spore 
;is exhausted. If, however, the spore germinates on the 
leaf of a wheat plant the germ tube growls to one of the 
stomata, and passing through the pore enters the tissues 
of the leaf. Here the fungus absorbing food material 
grows and produces a new pustule wath uredospores in 
about a fortnight. In this w^ay very large numbers of 
uredospores are produced during the summer months and 
hence the disease spreads rapidly. 

The teleutospores, like the uredospores, arise from 
branches of the filaments of the fungus in the leaf and 
they appear towards the end of summer in the same 
pustules as the uredospores. With the advance of the 
season, how^ever, the pustules give rise to teleutospores 
only. Microscopically, these are longer than the uredo- 
spores, are more spindle-shaped, but are also borne on 
stalks. Each spore consists of two cells and is furnished 
with a thick, resistant wall. Unlike the uredospores the 
teleutospores do not germinate immediately when placed 
tinder moist conditions, but they require to rest for a period 
of months. If, however, teleutospores which are produced 



92 



in the autumn are allowed to remain exposed to the 
weather throug-h the winter, they will, under suitable con- 
ditions, germinate in a few hours in March or April. In 
this case the process of germination of the spore differs 
from any of those dealt with in previous chapters. Each 
of the two cells of the spore sends out a delicate tube, 
which, after growing to two or three times the length of 
the spore, becomes divided into four cells or segments. 
Then a delicate peg-like branch is put out from each of 
these segments, and a minute, oval spore is formed at the 
tip of each peg. The question now arises as to what 
becomes of these teleutospores and the small oval spores 
or sporidia they produce. For many years this v/as not 
understood, although trials again and again proved that 
the teleutospores or their sporidia were unable to cause 
new infection on the wheat plant. 

The clue to the problem was discovered by De Bary 
m the old belief of farmers that outbreaks of rust were 
m some way connected with the presence of barberry. He 
first made a careful study of the yellow fungus which 
occurs on barberry, and then proved the connection of this 
with the rust on wheat. If one of the diseased areas on 
the barberry is closely examined it is found to arise on 
a swollen part of the leaf, and on the under side a number 
of small, yellow, cup-like bodies are produced. From 
these cluster-cups yellow spores, known as aecidiospores, 
are liberated as a fine dust. A section through the diseased 
area shows that the cluster-cups arise as round masses of 
fungal filaments beneath the skin of the leaf. Within 
the ball of fungus, aecidiospores are produced in chains^ 
and when this growing ball bursts the skin of the leaf, it 
opens at the apex and the edges turning back g^ive the 
structure the appearance of a minute cup or bowl. The 
aecidiospores arise from threads at the base of this, and 
as they ripen and are set free new spores are produced. 
Now^ De Bary discovered that when sporidia from the 
teleutospores on wheat are sown on barberry leaves in the 
spring, infection takes place and the cluster-cups just 
described are produced in two or three weeks. Not only 
did he prove this, but he also showed that when the 
aecidiospores from the cluster-cups are sown on the leaves 
of wheat, they germinate, and like the uredospores send 
g;;erm tubes through the stomata and thus infect the leaf. 
Pustules containing uredospores and later teleutospores 



93 



are produced from this infection. The connection between 
the fungus on the barberry and that on wheat was thus 
established, and it was shown that, although the appear- 
ances of the fungus on the two host plants differ so 
materially, each represents a stage in the life history of 
one fungus. 

Since De Bary's remarkable discovery very many 
rust fungi, showing this type of life history, have been 
studied. The rust which frequently appears on the leaves 
of the pear, for example, is the aecidiospore stage of 
Gymnosforangmm which produces swellings on the 
branches of juniper, where the teleutospores arise. The 
Blister-Rust of the Weymouth or white pine, a disease 
which is very prevalent on the continent of Europe, is the 
aecidiospore stage of a fungus w^hich forms its uredospores 
ajid teleutospores on the leaves of currants. Similarly, 
the Gooseberry rust is the cluster-cup stage of a fungus 
which forms the uredospores and teleutospores on the 
leaves of sedges. Many other similar cases occur as 
diseases of cultivated plants and trees. 

When the facts became known for the wheat rust it 
W'as thought that in order to eliminate the disease from a 
given district all that was necessary was to destroy the 
barberry plants in the district. It was soon found, how- 
ever, that in some countries, for example, Australia, where 
the rusts of wheat are most destructive, the barberry is 
almost unknown. In such cases it seems likely that the 
rust fungus is able to maintain itself without passing 
through the barberry. It probably does this by means 
of the uredospores, which have been shown to be capable 
of resisting a mild winter. 

Apart from the destruction of the barberry other means 
of combatting the wheat rust had to be devised. Up to 
the present this has proved a very difficult problem. The 
most satisfactory progress has been attained along the 
lines of breeding disease-resistant wheats. It has long 
been known that certain wheats are much more resistant 
to rust diseases than others. The chief difficult v lay in 
the fact that the most resistant forms, e.g., certain semi- 
w41d wheats, were almost valueless as crops. In recent 
years, however, much has been done, especially by Pro- 
fessor Biffen,of Cambridge, to produce by crossing, w^heats 
which combine qualities of rust-resistance with good 
cropping and milling capacities. The subject is neverthe- 



94 



less still beset with difficulties, both in regard to the 
permanence of the rust-resistance, and to the fact that 
a wheat, which is resistant to one form of rust may be 
equally susceptible to another. Further, wheats which 
are resistant, say in England, are not necessarily so in 
India or Australia. It has been essential therefore for 
each country to establish its own rust-resistant varieties 
of wheat. 

Many of our common garden plants are liable to 
attacks by different rust fungi. The Mint rust caused 
by Puccinia menthcE is one of the most prevalent of these 
in some districts. In this example the three forms of 
spore appear in succession, and there is no intervention of 
a second host plant m the life cycle of the rust. In the 
spring diseased plants send up shoots which are often' 
swollen and distorted, and bear the cluster-cups of the 
fungus. The aecidiospores are liberated as a bright yel- 
low dust and infect the leaves of healthy shoots so spread- 
ing the disease. The pustules produced by this infection 
are brown in colour and are scattered, as minute dark 
spots, over the leaves. Uredospores arise from these 
pustules throughout the summer, and towards the end of 
the season teleutospores are produced from similar disease 
spots. The teleutospores remain in a resting condition in 
the soil for some months, but germinate in the early 
spring giving rise to sporidia, which infect the young buds 
on the underground stem. Such infected buds are not 
killed outright by the fungus, but grow out to produce 
the distorted shoots bearing the aecidiospores, described 
above. It has also been shown that once the underground 
stem is infected, the fungus lives there perennially and 
grows into the young buds as they are formed. This 
renders the disease all the more difficult to eradicate. 
Indeed, the best plan is to destroy all infected material 
and only use for planting, rhizomes knowm to be free from, 
the fungus. Care should also be taken to prevent infec- 
tion by teleutospores from soil which has grown diseased 
plants. 

As an example of a rust caused by a fungus with a 
simpler life history, the Hollyhock rust produced by 
Puccinia malvacearum may be considered. This disease, 
occurs on a large number of the members of the Mallow 
family ; it is widely distributed throughout the world and 
is abundant in this^ country, both on wild mallows and on 



95 



the cultivated Hollyhock. The fundus, which is a native 
of Chile, was introduced into Europe about 1875, f6r 
some years its ravages were so severe that it was scarcely 
possible to grow the Hollyhock free from rust. Even at 
present the disease causes considerable trouble in some 
districts. 

The disease pustules chiefly occur on the leaves, but the 
stem and even the flower buds and fruits are often 
attacked. As in the rusts already described, the develop- 
ing cushion of spores bursts through the epidermis. The 
filaments of the fungus ramify in the tissues of the host; 
sending haustoria into the living cells, which are slowly 
depleted of nutritive substances. The pustules arise on 
both surfaces of the leaf, are small and circular in out- 
line and produce teleutospores only. These germinate 
m sitit under suitable conditions, immediately they 
mature, giving rise to sporidia as in other rusts. The 
sporidia, falling on to the surface of any part of a 
hollyhock plant germinate and penetrate the epidermis 
producing an infection. From this region a new^ pustule 
of teleutospores is produced in about fourteen days. The 
Hollyhock rust thus omits both the aecidiospore and 
uredospore stages from its life story. 

The fungus probably passes the winter by teleuto- 
spores which fail to germinate owing to unfavourable 
conditions. In addition it is likely that it also is carried 
over in the few radical leaves w^hich generally survive the 
winter in this country. The writer has frequently observed 
incipient disease spots on such leaves in the winter, and 
it is probable that these develop much m^ore slowly in the 
cold weather than those produced in the warmer months. 
By destroying diseased leaves as soon as the spots aopear, 
it is usually possible to restrict the damage caused b^^ this 
rust 

From the examples described in this chapter it will be 
evident that, although the fungi causing the rust diseases 
belong to one group, yet they present certain differences 
from one another. A large number, like thr;^ rust of wheat, 
produce aecidiospores on one host, and the iiredospores 
and teleutospores on an entirely different species of plant. 
Others, however, like the rust of mint pass the whole life 
cycle on one host and give rise to aecidiospores, uredo- 
spores and teleutospores in succession on the same species. 
Still others like the Hollyhock rust, in addition to living 



y6 

wholly on one host plant, altogether omit some of the 
spore forms from the life stor}'. Whilst showincr these 
variations, however, the rust fungi are all highly 
specialised parasites ; they cannot grow apart from living 
plants, and with rare exceptions, a specific rust fungus is 
restricted to a single species of plant. 

Further information on the subject of diseases of 
plants caused by parasitic fungi may be obtained from 
the following books: — 

Ward, H. M. Diseases of Plants," S.P.C.K., London. 
Duggar, B, M. Fungous Diseases of Plants,'- iqoq, Ginn aad 
Company. 

Massee, G. Diseases of Cultivated Plants and Trees,"' Dwck- 
wortb, igio. 



I 



Chapter 13. 

INJURIOUS ANIMALS OTHER THAN INSECTS. 

Injurious and beneficial Animals, Birds and the need for 
scientific investigation with reference to their food at different 
seasons of the year, Eelworms and more especially the Stem 
Eelworm and the Knot Root Eelworm. Pulmonate Molluscs 
(Snails and Slugs J. The Black Currant Gall Mite. 

All who engage in agricultural or horticultural pur- 
suits sooner or later have to concern themselves with some 
of the forms of animal life which are associated w^ith their 
plants. Very frequently certain of these animals are 
directly injurious to the operations of man, but there are 
others which, on the contrary,- are distinctly beneficial in 
their effects. It is, therefore, a matter of considerable 
importance to be able to discriminate between these two 
classes, for it is obviously bad policy to devote time and 
money in destroying organisms which are beneficial in 
their action. The animal kingdom comprises a vast 
assemblage of different forms, but fortunately for our 
present purpose we need only concern ourselves with a 
relatively small proportion of their number. These in- 
clude certain birds, species of Nematodes or Eelworms, 



97 



98 



Olig-Qchaeta or Earthworms, Pulmonate Molluscs or Slugs 
and bnails, land Isopods or Wood-lice, certain Acari or 
Mites, and the great class of the Insects. 

Dealing first with INJURIOUS ANIMALS. BIRDS (l), 
'2), *(3) merit some amount of attention, but the fact can- 
not be emphasised too strongly that we possess extra- 
ordinarily little reliable knowledge concerning the food 
of some of our very commonest birds. Both the British 
Association and the Board of i\griculture recognise that^ 
before any effective legislation can be recommended, a 
very full scientific enquiry is needed. It is necessary, 
for instance, to examine and tabulate the contents of the 
crops of certain common birds m each month of the 
year so that an opinion may be formicd of the benefits or 
injuries caused by them at the different seasons. It is 
further necessary that some estimate should be made of 
the available food in the district where the birds w^ere 
feeding when killed, in order to decide whether the foods 
discovered in the crops were selected from choice or 
necessity. Much information is also desirable as to the 
food of nestling birds. Fortunately some progress is 
being made towards supplying this much needed informa- 
tion, and the Department of Economic Zoology in this 
University is performing a useful part in the work on 
behalf of the Board of Agriculture. Certain species of 
wild birds may be direct-injurious by feeding upon or 
injuring plants or parts of plants; others are mdirectly 
harmful in that they may feed upon forms of animal life 
which are in themselves beneficial. Fortunately there 
are veiy few species of birds which we may declare to 
}>€ wholly destructive and, of these, the House Sparrow 
and Wood Pigeon are the most important. The Blacks 
bird also appears to have ver}' little utility in the eyes 
of man, and is a most persistent devourer of fruit. On 
the other hand, there is a large number of birds whose 
role is doubtful ; m many cases we lack adequate knowl- 
edge to ludge fairly as to their feeding habits, but they 
all appear to have a good deal of utility in their favour. 
Among these may be cited the Song Thrush, Great and 
Blue Tit, Greenfinch, Chaffinch, Rook, Robin, Linnet, 
Yellow-hammer, Skylark, Starling, Woodpeckers and 

^ The numbers in brackets refer to the literature which is 
listed at the end of this series of lectures. 



99 



others. The Rook, for instance, is a bird concerning 
which It IS at present impossible to say whether it is a 
benehcial or harmful species. It consumes a vast amount 
of grain but, on the other hand, during the summ.er it 
devours a great many injurious insects of various kinds, 
jncludmg both leather-jackets and wireworms. The 
Starling also devours a large amount of gram durm^j- 
certain times of the year, but this appears to be com- 
]>ensated by the great quantity of injurious insects which 
the bird consumes at other periods. As many as 197 
leather-jackets, for instance, have been found by ]\Ir. 
Leigh m the crop of a single bird. The Chaffinch is by 
no means as destructive as is commonly believed. It 
consumes large quantities of seeds of such troublesome 
weeds as, dock, knotgrass, hawkweed, and especially 
chickweecl. Mr. Leigh informs me that although he 
found grain m the crops of 41 per cent, of the birds which 
he examined, it appeared to have been taken m most cases 
from manure or ricks m farmyards and not from culti- 
vated land. It must be further added that evidence 
points to the fact that the majority of species of birds 
feed their nestlings on soft-bodied insects and other 
invertebrates. Consequently even the most destructive 
birds may perform a useftil function during that stage in 
their life. 

EelwoRMS '4 , '5 ., belong to the group of the Xema- 
toda. They are always small in size and have thread-like 
bodies, the two ends being more or less pointed. They 
can be readily distinguished from the Oligochaeta or 
Earthworms by the total absence of body rings or seg- 
ments. Those which are plant parasites are microscopic 
forms living free 111 damp soil or mside the tissues of 
plants. Others live m decaying vegetation, and both the 
parasitic and saprophytic forms can be recognised by the 
presence of a spme which can be proirucled through the 
mouth and serves to penetrate the cell-walls of plants. 
The Eehvorms spread from one plant to another by 
wandering through the soil, and when they leave the dead 
plants they lie near the surface of the ground. Fre- 
quently when these animals are numerous it is useless to 
grow susceptible plants m the same patch of soil during 
successive seasons, and then as long an interval as possible 
should elapse between the growing of two crops of the 
same plant. The STEM Eelworm {T ylenchus dei astatrtx,. 



lOO 



Kuhn; attacks a great variety of plants including straw- 
berries, onions, beans and peas, hyacinths, and also held 
crops. It is an extremely slender species, about i-2^ch 
of an inch long, and the males and females closely 
resemble one another. Strawberries when attacked decay 
away at the level of the soil or just below it, and the 
crowns and roots rot away. A remedy is to pull up and 
burn the affected plants and dress the soil with either 
lime or sulphate of potash m the proportion of i cwt. to 
the acre. The Knot Root Eel worm {Heterodera radici- 
cola, Greef) differs from the previous species in the male, 
being thread-like, while the female is greatly swollen 
except at the head end. It also goes through a more com- 
plex life-history. This species renders its presence evident 
by forming knot-like swellings or galls upon the roots of 
the affected plants. It is a great enemy of cucumbers .iiid 
tomatoes grown m glass houses, but also attacks vines, 
potatoes, lettuces, and many other plants in the open. 

As a temporary measure to save a growing crop, one 
part of permanganate of potash to 200 of water applied 
at intervals of ten days is recommended in the Kcw 
BuUetin. It does not harm the plants, but since it dc^s 
not destroy the eggs of Eelworms, no permanent value 
can be ascribed to it. Treatment of the soil with one part 
of carbolic acid to twenty of water, wath a dressing of 
sulphate of potash, 3 cwt. per acre, intimately mixing the 
soil with gas lime or naphthaline, are among the remedies 
that are recommended. When applying remedial measures 
the soil must remain unused for at least six weeks for any 
permanent benefit to be derived. This species, ho^vever, 
is extremely difficult to eradicate owing to the fact ;t 
produces vast numbers of eggs throughout the year, and 
the young Eelworms are thus constantly being liberated 
into the soil. Furthermore, most of the above methods are 
not lasting in their effects, owing to the fact that fre- 
quently a number of eggs remain over undestroyed, and 
serve to start the infection afresh. When a glass house 
is infested with Eelworms, it is often necessary where 
p>ossible, to remove the soil bodily and treat it by one 
of the methods already mentioned. In the case of plants 
grown under glass the horticulturist soon finds that the 
conditions encourage a host of other living things. In 
addition to Eelworms, Woodlice, insects of various kinds, 
and fungi often enforce their presence, and under the 



lOI 



warmth and moisture that i>. provided they are hable to 
multiply exceedingly. Experiments carried out at the 
Rothamsted Station have shown that we can very con- 
siderably reduce this undesirable population by partial 
sterilization of the soil by means of steam. In cases of 
very bad infestations of Eelworm this method is said to 
be the only effective remedy at present available. 

_ Slugs and Sis-ails (6),^ belong to the class of the 
Zvlollusca, which is a large assemblage of animals includ- 
ing such divers forms as Oysters, Whelks, Scallops, Octopi, 
and the familiar fossils which are known as Ammonites 
a.nd Belemnites. Both Slugs and Snails differ from other 
Alolluscs in being land and not aquatic animals. They 
are always provided with a pulmonary chamber, which is 
a kind of lung enabling them to breathe in the air. In 
aquatic ?\Iolluscs this pulmonary chamber is almost always 
absent, respiration taking place by other means. 

Snails or Hehcidae are provided with an external 
spiral shell into which the animal can withdraw itself, and 
there are three species which are commonly met with. The 
Garden Snail (Helix asft!rsa, ]\Iull.) is the largest and 
its shell measures about lA inches in diameter. It is well 
enough known to need no description, being easily recog- 
nisable by its browm shell marked with pale irregular 
lines. The Strawberry Snail [H. rnlescens, Pen.) has a 
shell which seldom exceeds half an inch m diameter and 
is more flattened m form. It also varies in colour from 
dirty grey to brown or reddish-brown, often with a num- 
ber of transverse streaks of a darker tint. The Wood 
Snail neinoralis) has an extremely variable shell being 
white, grey, pinkish, yellow or brown, and is marked w^th 
one to five or more conspicuous browm spiral bands. It 
is, moreover, considerably larger than the Strawberry 
Snail. 

Slugs or Limacid^ are naked and only possess a 
vestigal shell, which is placed near the hinder end of the 
body or buried beneath the skin of the back; all the 
iniurious species have the shell m the latter position. The 
situation of the shell is clearly marked externally and 
the area of skm covering it is known as the shield or 
mantle. Closely related to the margin of the latter, on 
the right side of the body, is the respiratory pore — a well- 
defined aperture leading into the pulmonary chamber. 
Slugs secrete an abundance of mucous, which serves to 



102 



lubricate the skm; it is very tenacious and capable of 
being drawn out into a diread which is used as a means 
of descent from trees and bushes. The most miurious 
species are: (i. The Black Slug \A.rion ater, Linn.' • not- 
withstanding its name this species varies greatly in colour 
and may be either black, grey, reddish, or reddish brown. 
When at rest the animal can be further recognised by its 
contracted and almost hemispherical form. '2^: The Grev 
Field_ Slug [Agnoliinax agrtstis, Lmn.^ is perhaps the 
most injurious species we have in this country. It is ashy- 
grey m colour with a yellowish or reddish tinge, and occa- 
sionally specimens have a mottled appearance; longitu- 
dinal markings are entirely absent. '^3^ The Black Striped 
Slug \Liniax iiiaxinuis, L.; 13 the largest of the three and 
may attain a length of over six inches. It is usually some 
shade of grey, with longitudinal markings of a darker 
colour, frequenth' black. Individuals inclining to brown 
or dull yellow are also not infrcvquently met with. 

Tc is well known that both Slugs and Snails coniine 
their operations to night, and are seldom evident during 
the day except after rain. It is consecjuently useless to 
apply remedial measures during the warm parts of the 
day, or in very dr}' weather, the evening and early m.om- 
ing being the most suitable times. The mucous secreted 
by Slugs enables them to resist the action of obnoxious 
substances m the powered condition, they have the faculty 
of crawling out of their mucous investment, and m that 
vray leave the powdered material behind them. This 
mucous, however, cannot be secreted indefinitely, and if 
two or more dressings are applied with an interval of 
about fifteen minutes between each application the Slugs 
are usually killed. A mixture of lime and soot applied 
two or three times is an effective remedy, but the lime 
should be quite fresh and very hneh" povrdered. Accord- 
ing to Theobald the most effective substance is hydro- 
oxide of calcium, a i to 2 per cent, solution m water. 
Snails, on the other hand, are more difficult to destroy 
from the fact that they retract themselves into the shell 
and can close the mouth of the latter. \n this condition 
they can remain completely dormant for several vears. 
Dressings of soot is a well-known remedy against Snails, 
it acts as a deterrent making the plants and surface of 
the soil obnoxious to these animals. Xitrate of soda is 
an effective dressing for use on a large scale against both 



103 



ai~a ?nd, morco\-er, is benelicial to the plants. 

Xaiurc:; : also an important factor: thrushes, 

blackbirr.-^ -^arl::j^\-, Lud also ducks and tOvvl render help 
in k^-:-p::u- down an excess of Slugs and Snails. 

/i:-::::-, other injurious animals WOODLICE 'S^ and 
EDE-- '7 were also reierred to but, owing- to the 
hiixiiia -pace at my disposal, I miist pass over these and 
deal with the Acari or }vIitls. 1 hey are classified as a 
group of the Arachnida, v.iiich also includes Spiders, 
Harvestmen, and Scorpions. All can be recognised by 
the presence of eight pairs of legs, the absence of feelers 
or antennae, and the fusion of the head and thorax into 
one compact region or cephalothcrax. Acari are further 
distinguished oy the abdomen no-t being clefi.nitely marked 
off from the rest of the body. The Red Spiders or Trom- 
bididae belong to this group, but the most important for 
our purpose are the Eriophyidae or Gall ]\ntes. Erio- 
phyes ribis \\\ or the BLACK CURRAXI GaLL AIitE is 
responsible for the Big-Bud " disease which has spread 
throughout the country. lis presence can be readily 
detected by the swollen and distorted apoearance of the 
buds which harbour the Alite. Badly infested buds sel- 
dom de^relop into shoots, they remain wnc^tened and, after 
retaining their green coeiv.r for a iin:-:, oeeome brown and 
die off. The damiage is caused by the jaws of the ^Iite 
cutting through the e'jidermis of the delicate young leaves, 
followed by the m-ertin^ of the sucking tube which 
extracts the sap. Throughout the winter ^he jJite- teed 
and shelter in the galled buds. Aiigraticn :?.-:_s r rrom 
the infected ouds, which open from aoout the muddle of 
April unril well o-n in June. The Antes then crawl out 
m great numbers in order to hnd new and succulent buds 
to -erve for their future sustenance. This migration is 
aided by the habit the Alites possess of often attaching 
themselves to passing insects wandering over the twigs. 
By this means they become distributed to other branches 
and to fresh bushes. Strong winds are also a factor aid- 
ing their dispersal. Having entered new buds the ]^Iites 
commence laying their eggs and thereby multiply rapidly 
until the end of the summer. Shoots examined during the 
end of August and m September, exhibit the ''Big-Bud" 
appearance, and are filled with the new generation of the 
:\Iite, which vsuU carry on infection for the next season. A 
certain number of eggs are to be found all the year round 



I04 

but are most abundant m the summer. Our knowledge 
of the hfe-history of this ]\Iite is incomplete, we still 
require definite information as to whether the species can 
pass the winter elsewhere than m the buds — whether it can 
survive under the bark, m the roots, or beneath the soil. 

With regard to remedial measures, so far as I am 
aware, no completely successful methods of treatment have 
yet been devised. Instances are known where all diseased 
bushes in a plantation have been cut down, the stumps and 
root stocks subjected to treatment, and yet the young 
shoots came up infested with this ]\Iite. It is of first 
im^portance to cultivate from perfectly clean stoek, and 
cuttings taken for setting should also be selected from 
such plants. Hand picking of the infected buds at the 
end of winter is valuable, and all buds collected should 
be burned as soon as possible; with badly infected shoots 
extensive pruning is necessary. When the bushes are very 
badly infected there is no remedy beyond taking up and 
burning them, followed by replanting with clean new 
stock. Spraying or dusting with a mixture of lime and 
sulphur during the migratory period has been recom- 
mended, but often the results are unsatisfactory. An 
efficient spraying mixture still remains to be discovered. 
Some varieties of currant are claimed to be less severely 
attacked than others, and among them may be mentioned 
the Boskoop Giant, Lee's Prolific, and Edina. Varieties 
claimed to be immune have been placed on the market, but 
whether they will remain so time alone will determine. 
There is a 'possibility that careful selection and inter- 
crossing of likely varieties along scientific lines may lead 
to the production of resistant stock. 



Chapter 14. 



INJURIOUS INSECTS, 

General observations on Insects and their feeding habits. 
Insecticides and their use. Earwigs. Destructive Caterpillars 
of Cabbage Butterflies^ and the Currant, Winter, and Codling 
Moths. The Gooseberry Saw-fly. The Pea Thrips. Wireworms. 



The next class to be considered is that of the IXSECTS 
(g\ lo),'-^ and, viewed from our present standpoint, they 
are of greater importance than the whole of the rest of 
the Animial Kingdom. Insects can be readily recognised 
by the presence of a pair of antennae or feelers, six pairs 
of legs, and the division of the bod}' into head, thorax 
and abdomen. Either one or two pairs of wmgs are almost 
always present in adult Insects. The most remarkable 
feature m their life-history is the fact that they pass 
through a series of changes which we term metamorphosis. 
On hatching fromi the egg, the first stage is the larva, 
which is succeeded by that of the pupa, and from the 
latter emxcrges the perfect Insect. In many Insects, how- 
ever, the pupa is absent, and then there is a gradual growth 
from the larva to the perfect Insect. Larv^ are variously 
known as caterpillars, maggots, or grubs, and, being 
exclusively concerned with feeding, they are as a general 
rule more destructive than the perfect Insects. The pupa 
or chrysalis is purely a resting stage, no food is taken, 
and during this period the organs and tissues of the future 
Insect are gradually built up. It is of great im.portance 

- The numbers in brackets refer to the literature given at the end 
of the final lecture. 



105 



io6 

to understand the method by which an Insect feeds, 
whether it be m the larval or adult condition. Almost all 
remedial measures have to be based upon this factor. We 
can recognise three methods by vsiiich Insects feed: 'i) 
By means of the bitmg action of their jaws. [2) By means 
of sucking, and m this case the mouth organs are mcdihed 
to form a suctorial apparatus. 3) By iPxeans of a comi- 
bination of both of these methods. 

Various chemical substances are used for destroying 
Insects and are known as Insecticides. Bitmg Insects are 
mainly destroyed by poisoning their food, while sucking 
Insects can usually only be destroyed by using contact 
insecticides — those which kill by means of surface contact. 
This method is adopted for the simple reason that we 
cannot poison the food when it consists of the internal 
juices of plants, as is the case with sucking Insects. When 
using insecticides the grovv^er should proceed with caution 
until experience has been acquired. Also contact insecti- 
cides are liable to injure the foliage under certain con- 
ditions. ]\Iany of the insecticides that have been recom- 
mended are mefhcient, while others need scientific testing 
to fully determine their value. Insecticides are artiricial 
7]iethods of control, but it is necessary to point out that 
the utilisation of natural methods of control should not be 
neglected. Natural methods consist in the preservation 
and increase of those organisms which are directly bene- 
ficial to man, in that they destroy the injurious forms of 
animal life. This latter method will be dealt with m a 
subsequent lecture. 

The first order of Insects that I shall deal with is the 
Orthoftera, and the only member thereof that concerns us 
is the common Earwig [Fcrficula aiirtcularia) \ io\. It is 
a good example of an Insect which undergoes incomplete 
metamxorphosis. The adult Earwig lays its eggs a 
group either beneath stones or in the soil. _ During the 
incubation period she guards the eggs until they hatch, 
exhibiting in this respect a rudimentary mstance^ of 
parental care. The young larva=^ are minute white 
creatures, with very slender forceps and no traces of wings. 
After they have passed through several moults rudiments 
of wings' appear, and subsequent growth chiefly consists 
of an increase in the size of the Insect and the gradual 
development of the wings. Xo pupa or resting stage is 
passed through. The Earwig is almost exclusively noc- 



107 



turnal in its habits, and has been very seldom obser\-ed 
to use its wings m flight. During the clay Earwigs hide 
away beneath the soil, among vegetation, under stones, 
bark, and in other dark situations. They cause some 
amount of harm to cultivated plants, particularly dahlias, 
but their diet may include animal matter also. Earwigs 
can be most readily got rid of by means of traps. Plant 
pots filled w^ith straw or dead moss placed m an inverted 
position upon the ground, or upon stakes, are usually 
effective. The pots should be examined frequently, and 
the Insects shaken out into boiling water, or the straw and 
other rubbish containing them burnt. 

The next order of Insects which concern us is the 
Lepicioptera. Their larv^ are known as caterpillars, while 
the perfect Insects are recognised as Butterflies and Moths. 
Butterflies can be readily distinguished from Moths by 
their feelers or antennae terrnhnating in a club or knob, 
while those of Moths taper off to a point. Furthermore, 
Butterfl^ies are diurnal while most Moths are nocturnal. 
Lefidoftera are only injurious in the caterpillar stage; 
the adults feed entirely upon the nectar and juices 
of flowers which they imbibe by means of a flexible 
sucking tube, and never pierce or injure the tissues of 
plants. Four wings are present and they are closely 
covered with microscopic scales w^hich easily rub ofr, 
revealing the transparent wing membrane beneath. The 
eggs of Butterflies and ]\Ioths are almost always laid on 
or near the leaves of the, plants which are to serve 
as food for the future caterpillars. Very few Butterfly 
larv^ are injurious, and only two species need con- 
cern us, viz., the Large and Small Cabbage Whites 
{Pieris brassicae and P. rafae) (lo). The larvae ol 
the former species are partial to the outer leaves of 
cabbages, while those of the smaller species also attack 
turnips. Both kinds frequently devour the leaves of 
" nasturtiums " and other plants. When very abundant 
hand picking of the larv^ is the best method. The pupa? 
occur on palings, walls, and similar objects in the imme- 
diate vicinity, but a quick eye is needed to distinguish 
them, and for this reason their destruction is not likel^^ to 
very materiall}/ reduce their numbers. The eggs of the 
Large Cabbage Butterfly are pale yellow and laid m 
clusters on the under sides of the leaves of the food plants. 
Every cluster destroyed means the reduction of a whole 



io8 



brood of the larv^. The eggs of the Small Cabbage 
Butterfly are laid singly and, consequently, their detection 
and destruction is too laborious to be worth while. 
Insecticides are of very little value against these two 
species. Among [Moths, the larvae of the common Currant 
^vloih ^^Abraxas grossvlariatd) (4) are very destructive in 
that they defoliate currants and gooseberries. The ]vIoth 
is conspicuously spotted with black on a white ground, 
and IS on the wing during July and i\ugust. The larva 
is similarly conspicuous, being deeply spotted with black 
on an ochreous-Vvdiite ground, with an orange coloured line 
along each side. It is prevalent at the end of the summer 
and hibernates during the winter among dead leaves, in 
chinks of walls, under bark, etc. During the spring it 
recommences feeding and turns to the pupa in I\Iay or 
June. The pronounced colouration of this larva renders 
hand picking- a very easy and effective m.easure. In exten- 
sive infesta-tions spraying with lead arsenate at the end 
of the summer kills large num^bers of the young larvae 
through poisoning their food. If they are still abundant 
during the following spring the operation should be 
repeated. Since lead arsenate is a poison it must not be 
applied later than four weeks before the fruit is to be 
gathered. The grower will do best to utilise Swift's 
arsenate paste rather than prepare his own compound. 
From 8 to loozs. of the paste mixed in 10 gallons of 
soft water is a suitable strength; weaker solutions, how- 
ever, are often equally effective. The Winter ~\lo\h 
{Cheimatohia bniniata) (4) does immense damage to the 
foliage of apple, pear, plum and cherry trees and is univer- 
sally common in this country. The male is a thin-bodied 
brown Moth, measuring ijin- across the expanded wings. 
The female is wingless and spicier-like in appearance. 
The Insect occurs from October until the beginning of 
January, the eggs are laid on or near the bases of the 
buds, and the green larvae belong to the type commonly 
known as loopers." They commence feeding upon the 
leaf buds, and then the flovrer buds, which they spin 
together with the leaves to form shelters. Later on they 
attack the foliage and even the fruit. During June they 
are fully fed and pass to the soil, where they change to 
the chrysalis a few inches below the surface. The most 
effective measure is " grease banding " the trees.^ Strips 
of grease-proof paper, 6 to S inches wide, and sufficiently 



log 



long to encircle the trunks, should be tied tightly with 
string above and below, and placed on the trees during the 
first week in October. The most suitable height is from 
2 to 4 feet from the ground. The paper is to be well 
smeared with cart grease, which must never be allowed to 
become dry. To ensure this, three apphcations during the 
season are usually sufhcient. " Tree tanglefoot " may be 
used instead of cart grease, and has the advantage of not 
requiring renewal during the whole winter. By means of 
this device the Vv-mgless females are trapped in large num- 
bers as they crawl up the tree trunks from the soil. If 
the grease bands be retained until the end of ^larch, large 
numbers of femiales of the destructive IMarch Moth 
' Aniso fiery X aesciilana , ^4}, which are likewise wingless, 
also meet with a similar fate. If rhe grease bands are 
neglected many of the female ]vIoths succeed in making 
their way up to die buds to lay their eggs. AVhen the 
larvae are very abundant the only measure is to spray with 
lead arsenate, using an orclinar}' knapsack sprayer, except 
for verv lar^-e trees, which demand a more powerful instru- 
ment. T'he spray should be distributed as a nne mist, 
as all that is needed is to render the lea^:es poisonous. 
It IS not advisable to spray during blossoming, and spray- 
ing with winter washes is useless. The Codling "\Ioth 
[Carpocapsa fonionelLa 4) is one of the most important 
of apple pests, attacking many varieties besides the Cod- 
ling, and also pears. Those types such as the Russet and 
Xonpareil, in which the eyes are more or less closed, 
are less susceptible tlian the Blenheim Orange and many 
others. The perf:-ct Insect is a pretty brown ]\ioth with 
coppery reflections, and measures abotit fm. from tiD to tip 
of the expanded wings. It flies during June and July, 
laying its eggs singly on the young fruit, but occasionally 
it may select the leaves. They hatch just about the time 
when the petals have fallen and the fruit set. The young 
larv^ are whitish, pale a'cIIow. or often pink, with the head 
and the shield immediately behind dark brown. They 
make their way to the calyx end of the fruits and 
gradually eat their way to the core. The entrance hole 
can always be detected, and through it the larvse ejects 
particles of excrement to the exterior, thereby avoiding 
contamination of its burrow. About midsummer they eat 
their way out of the fruit, and if the latter are still on the 
tree the larvae crawl down until thev reach the trunk. In 



I lO 



the case of fallen fruit the larv^ make their way back to 
the trees and crawl up the trunk. In either case when 
the trunk is reached they spni cocoons among loose bark, 
moss or lichens, and there remani dormant until the fol- 
lowing spring, when the}' turn to the pupa and shortly 
afterwards give rise to the next generation of Aloths. In 
a few instances two broods have been noticed to occur in 
one year. Ihe attacks of this Insect cause the fruit to 
fall prematurely or decay rapidy when stored. As a 
remedial measure all loose bark, moss, etc., should be 
scraped ofl' the trunks, and artificial shelters in the form 
of one or miore bands cf loose straw or old sacking should 
be tied round the trees, not very far from the ground. It 
is safest to do this in June, and the bands can be examined 
at leisure during the v.'inter and burnt. Bv this means 
large numbers of the cocoons containing the larv^ are 
often destroA'ecl. Fallen apples should be cleared away 
as soon as possible. Lofts and rooms utilised for storage 
should be v\-ell swept out, and the walls, floors, shelves, 
and vrmclow frames lime-washed. In severe attacks spray- 
ing the fruit-bearuig portions of the trees with arsenate 
of lead is advisable, and should be carried out a few davs 
after the petals have fallen. The larv^ have to eat the 
coating of this mixture in order to make their way into the 
calyx, and are poisoned thereby, it the application has been 
successful. 

The order Hymenoptera is characterised by the presence 
of (i) tvro pairs of transparent vun^s provided with rela- 
tively few veins. '2) biting and sucking mouth organs, and 
;3) com^plete metam.orphosis. The Sawflies are the onlv 
group that directly concerns us, and they may be easily 
separated from, other H}.mienoptera b}' the absence of a 
'''' v\'aist/' or constriction of the body. The Gooseberry 
Sawfly '}seiiiai::s ribesiv (4), (10'^, is very destructive to 
red currants and gooseberries, but seldom harms black 
currants. The perfect Insects appear m April and I\ia}/; 
they have yellow bodies marked veith black, and measure 
abo'ut fin. 'in wing expanse. The eggs are laid in neat 
rows along the veins on the undersides of the leaves of 
the host plant. They hatch into bluish-green caterpillars 
spotted with black, and also marked laterally with blue 
and yellow. Unlike 3.Ioth caterpillars they possess ten 
oairs'of feet, and when fully grovai measure about fm. 
long. The bushes may be very quickly stripped of their 



1 1 1 



foliage by these larva", and the fruit are not exempt from 
attack. About the begmnmg of June, they enter the soil 
beneath or near the bushes ui order to spin their brown 
papery cocoons ^vithin which the pupal stage is passed. 
Fromi ten to twenty-one days, according to the tempera- 
'ture, are passed beneath the ground until the flies emerge, 
and there are three broods during the year. The autumn 
larvc^ pass the winter in their cocoons, turning to pupae 
early the following spring. Leaves bearing the eggs of 
this Insect should be destroyed whenever m.et with, while 
hand picking is an effective micans of getting rid of the 
larvae if done thoroughly. In the autumn wholesale 
removal of the surface soil beneath and around the bushes 
to a depth of hve inches is valuable. It needs to be buried 
in a deep hole dug for the purpose. By this means the 
winged Insects are buried beneath the earth and perish on 
emergence from the pupae. Fresh soil and m.anure should 
be placed round the bushes. Spraying with arsenate of 
lead is an effective poison for the larvae, and can be applied 
any time they are abundant after the fruit has been 
gathered. 

The Thysancptera form a very small order of Insects, 
comprising only those minute forms which are known as 
" Thrips." They are provided with four strap-like wmgs 
with long fringes" all round, and are entirely suctorial 
in their feeding habits. The Fea Thrips [Kakothrifs 
rob?istus) (ii) IS a dark brown Insect, about -xVi^^- long, 
attacking edible peas and broad beans., often causing much 
dam.age. The adults occur from ^Jay until August, and 
the eggs are laid within the flowers on the stamen sheath 
or on the ;.'Oung developing pods. The larv^ resemble 
the adults with the exception of having no wings ; when 
fully fed the\- descend to the ground, penetrating to a 
depth of 3-1 2in. They remain in the soil until spring, 
vrhen the adults emer^;e from the pupae, there being thus 
only one brood in the \-ear. Both the larvae and the adults 
are iniurious, and ni bad attacks no pods are formed. or 
are curled and undersized. The terminal buds and shoots 
may also be infested, and damage is stated to be most 
severe m light soil. This Insect sometimes spoils a whole 
crop, amd no varieties appear to be immune, but it has not 
so far been found on sweet peas or scarlet runner beans, 
though they are mentioned as host plants m France. Con- 
trol IS difficult to achieve, but early sown plants are less 



112 



severely attacked. Spraying- is useless when the Insects 
are in the flowers as it does not reach them in that situa- 
tion. When they feed openly on the pods in large num- 
bers, spraying by means of contact insecticides is then 
likely to be effective. A mixture of i lb. soft soap m 
10 gallons of water is a cheap remedy and worthy of a 
trial, or better still 3 lbs. tobacco powder (or i lb. of Voss 
tobacco extract;, i lb. soft soap in 10 gallons of water. 
Treatment of the soil during the winter does not offer m^uch 
promise on account of the depth to which the Insect 
descends. 

Coleoptera or Beetles are characterised by the anterior 
pair of wings being modified to form horny sheaths which 
usually cover the upper side of the abdomen. They are 
exclusively biting in their feeding" habits and pass through 
a direct metam.orphosis. Wirew^orms (12) are the larvae 
of Click Beetles (family Elateridae) and are known to 
attack almost any kind of crop. They are more especially 
pests of the agriculturalist, though tomatoes, strawberries, 
potatoes, and other vegetables are liable to suffer injury 
when grown in gardens or allotmients. Three years and 
even more are believed to be spent in the larval stage and, 
owing to their resistant coats, these larv^ are notoriously 
difficult to destroy, no effective remedy having so far been 
discovered. Lures in the form of slices- of potato, carrot, 
or beet buried an inch or more beneath the soil vvhen Wire- 
worms are prevalent, often attracts considerable numbers 
which can then be readily destroyed. The lures should 
be examined twice a week and the spots marked with pegs. 
In bad infestations crude powdered naphthaline dug well 
into the soil in the autumn or early spring is worthy of 
trial. Gas lime, lime, or salt are of very little use. In 
the case of a badly infested potato crop there is no remedy 
beyond digging it up. Infested soil should be well turned 
over, exposed, and broken up. Birds then have easy access 
to the Wireworms and material benefit is often derived by 
adopting this measure. 



Chapter 15. 



INJURIOUS INSECTS [Continued). 

Crane Flies; the Pear Midge; the Celery Fly; the Cabbage 
Root Fly; the Onion Fly; the Narcissus Fly; the House Fly. 



In this lecture we are concerned with the order Dipt era 
which comprises the true Flies. These Insects can be 
recognised by the presence of a single pair of wangs, the 
hinder pair being absent, and only represented by curious 
knobbed organs known as halteres or balancers. The 
larvae of the Diptera are devoid of true limbs and are 
commonly known as maggots. A pupa stage is always 
present and, in a very large number of species-, the skin 
of the larva is retained, forming a hardened case or 
puparium enclosing the true pupa. Although no aclult 
Flies are directly injurious to vegetation, certain kinds 
such as Mosquitoes, Sand Flies, and Tsetse Flies are 
injurious to man. They pierce the skin in order to suck 
his blood and thereby act as carriers of the organisms of 
some of the most virulent diseases. 

Some of the most familiar of the larger Flies are the 
Daddy Long Legs,'' or Crane Flies {Tifula oleracea and 
allied species) (13). Although they are commonly pests 
of our meadows and cereal crops nevertheless they not m- 
frequently injure turnips, peas, beans, cabbages, hops, 
dahlias, carnations and odier garden plants. Their larvae 
are commonly known as leather jackets,'' and when fully 
grown they attain a length of i|in. In colour they are 
dull grey or brown and are not unlike fragments of small 



113 



114 



dark :v'igs. The}' live exclusively beneath the soil and, 
although they devour a considerable amount of dead 
vegetable matter, their staple diet seems to consist mainly 
of the roots of various plants. They are specially conmion 
m damp parts of meadovv^s. vdierever there is rank herbage, 
especially grass. Leather Jackets feed mostly at night 
vrhen they often come to the surface of the soil. W hen 
fully fed they turn ro elongate pupa:-, \vhich iorce their 
Wcr- ''"ice of the -oil, where the}' may be often 

se;:_ - abc'Ut half their length verticallv out 

of the grcvnd, t ' ttn; r lies issue late in the spring 
and la}' their Oia,..t tndle--haped eggs on or near the 
surface of the ground. These eggs gi^'e rise to the Leather 
Jackets which e\'eiituall}' transform into a second brood 
of Flies appearing m great numbers during August and 
September. Tli? lace brood of Crane Flies is ahways more 
abundant btt th^ -nring on^, their eggs develop into 
lar^'te v;hi ■ u all through the winter. 

The}' are t v, _u jui:cia]iL ; - ^~ ^^t"iis in low-lying 

dist^'ict^. and it is advisabL - - to roll heavily 

an^' "' ;ass clc= , Loliing at the proper 

sea: 1 ihe pupa: . " done regularly after dark 

a large iramber of the laf .ild probabl}' also be 

destro}'ed. "When numbers thorough turn- 

nig of the soil lu '^ k .'"inter renders the larv^ 

accessible to rook.^. -lar:: ^ taid other birds which prey 
upon them m large nun ' A 9:cod soil dressing is 

I to 2 cwts, of nitrate c : - " acre, and although 

Leather Jackets are stisc . _ - titects. the}' are by 

a t atean- ak."a}'S eradicated. Gas lime is onh' doubtfullv 
enective. Thecbald advi-'f-s the use of traps of partiallv 
buried turf as a device ftt viticing the Flies to la}' their 
eggs, and aLo to attract the larvae from the adjacent soil. 
To arrt.rt itcal attacks m parts of lawns and beds i oz. of 
carbon bisulphide to each Svquare }'ard injected b}' means 
of a Vermorel injector, or other -uitable instrument, to a 
depth of about 6 inches is usually quite effective. 

The Pear klidge [Di-plosi: fyrrjor ' ' \ is one of the 
worst enemies of pear erov/ers. All varieties appear to be 
attacked b}' thi= Lisact. I'ait it is not known to affect any 
other kind ; ^^;"ut. The adult tvlids^e is onh' about iin. 
long and :kish-o-re}' or black in colour; the female 

can be di-ruicpushed from the male b}' the abdomen ter- 
minating in a long pointed egg-la}'ing instrument or 



115 



ovipositor. The Flies or Midges first appear during April 
jusL about the time when the pear blossoms commence to 
show their petals, and are to be found up to about the 
•Txidclle of Ma}/. The eggs are laid within the blossoms, 
and when the latter are unopened the petals are stated to 
be pierced by the ovipositor and the eggs deposited on the 
anthers. In opened flowers the pistil is pierced and the 
eggs inserted therein. The larv^ are very minute w^hite 
or pale yellow^ maggots and only attain a length of about 
i m. w^hen fully grown. They feed within the develop- 
ing fruitlets eating out their centres and leaving behind 
them a mass of decomposing tissues. As m^any as ten, 
twenty, or even thirty of these maggots may be found 
wdthin a single fruitlet. The attacked fruits usually swell 
more rapidly than sound o-nes, and can be readily recog- 
nised on the tree by being often twice the size of the latter 
and more or less distorted. When mature the maggots 
leave the fruitlets either before or after the latter have 
fallen. In either case they crawl out from their shelter 
and exhibit curious jumping movements until they bury 
themselves in the ground beneath the trees. Here they 
spin delicate cocoons of a dirty creamhsh colour, and 
hibernate therein throughout the rest of the year until the 
following spring. Unfortunately there is no universal 
measure for dealing with this Insect. All infested fruit- 
lets should be collected and destroyed before the larvae 
leave them. In very bad infestations it is better to gather 
and destroy the whole crop. If an orchard be w^ell stocked 
with poultr}/ in the spring w^hen the Flies appear and also 
in June when the maggots reach the earth, mxaterial benefit 
is very often attained. Removal of the surface soil con- 
taining the pupcK is scarcely a practicable measure. In 
America the application of Kainit is recommended. If 
w^ell spread at the rate of half a ton to the acre around the 
trees it is stated to destroy the larv^ and pup^ in the soil. 
When the larvae are leaving the fruitlets 5 cAvt. to the acre 
is said to be sufficient to destroy them.. In this country 
Kainit has been very little used, and reports as to its value 
are conflicting, nevertheless, it fully merits a fair trial. 

The Celery Fly (Aciclia heradez) (17) is a pretty 
brownish Insect with mottled ornamental Vv^ings. It 
may appear at the end of April but is commonest in 
June and there are several broods in the year. The eegs 
are laid on the leaves of celery and also parsnips. The 



ii6 

larvse are white or greenish maggots which mine the leaves 
of those plants. There may be several larvae in a leaf and 
by their devouring the middle layers of tissues transparent 
patches result, covered only by the upper and lower 
epidermis. After a while these patches become browm and 
the functions of the leaves are very greatly reduced. The 
larvse turn into yellow^ or yellow-brown pupa:^ shaped some- 
what like minute barrels, and are found sometimes in the 
leaves but mostly in the soil. The Insect w^inters in the 
pupa buried a few inches beneath the ground. This fact 
is of value wdth regard to preventive measures and deep 
trenching in winter betw^een the original row^s and burying 
the soil therein containing the pupae will destroy many 
of the Flies w^hich w^ould otherw-ise emerge. The mixing 
of gas lime with the soil adds to the effectiveness of this 
measure. Screening the young plants with cheap muslin 
when first put out protects them from the Flies until they 
are w^ell established, and less liable to suffer severely from 
the Insect. Picking off and burning the mined leaves will 
destroy the larvae, but in bad attacks the depletion of the 
foliage by this method would be too great. All infected 
celery tops should be burnt and not cast aside on refuse 
heaps. Theobald recommends spraying with nicotine; a 
useful formula is i oz. of 98 per cent, nicotine, and ^ lb. of 
soft soap to 10 gallons of water. Various preparations of 
nicotine are obtainable and so long as the above propor- 
tions are maintained it matters very little which is used 
(15). It is best to spray in the evening and when the 
foliage is not too dry, the spray is said to soak through 
the epidermis and kill a large number of the larv^. 

The Cabbage Root Fly {Chortofhtla brassicae) (7' is 
one of the worst pests of cabbage and cauliflowers, and 
may also attack radishes, turnips, swxdes and stocks. 
Growth of the affected plant is checked, the leaves flag and 
discolour, the roots are largely destroyed, and the plants 
die. The Fly is an ashy-grey Insect not unlike the House 
P ly in general appearance and measures about 4in. long. 
The winter is passed through in the pupa stage and the 
first brood of Flies appear in April or the beginning of 
May and there are most probably three generations in a 
year. The eggs are visible to the naked eye, and are laid 
close to, or on the plant, usually iust below the surface of 
the soil. The larvae are typical Fly maggots,, w^hite or 
pale yellowish, and about Jin. in length when mature. 



117 



They commence injury by gnawing the outer layers of the 
young roots, afterwards making tunnels inside the main 
root; they may also invade the low^er part of the stem. 
The pupas are about 4 in. long, oval in form, a light or 
dark brown and are found in the soil close to the plants. 
x\s regards preventive measures early plants have the best 
chance of success as they become well set before the bulk 
of the Flies appear. Earthing the soil around growing 
plants IS valuable as it causes the development of fresh 
rootlets, v\iiich serve to replace those already destroyed 
by the maggots. A cupful of paraffin well mixed with 
each bucketful of sand, sprinkled round the plants once 
a week until good growth is made, is to be recommended 
and it acts as a deterrent, driving the Flies elsewhere. 
Dusting the young plants with soot is said to be effective 
and is well worth trial. In Aimerica tarred felt paper discs, 
slipped round the stems of the young plants and pressed 
fiat on the ground are strongly recommended. They are 
said to afford eflicient protection to young plants against 
the Flies laying eggs thereon. Experiments are being 
conducted under my direction to test the value of these 
discs, and if they prove satisfactory, their low cost and 
the simiplicity of the method will argue strongly in their 
favour. \\^hen the crop is infested m^uch benefit is derived 
by pulling up and burning all infected plants as soon as 
noticed. Furthermiore, all cabbage stumps should be up- 
rooted straight away and not left to decay; by these 
means large numbers of larv^ are destroyed which would 
otherwise escape into the soil to pupate. In very severe 
infestations I vvould strongly advise discontinuing grow- 
ing cabbages for one year, and replacing with peas and 
beans or other distantly related crops. Unless some such 
course be taken bad infestations may continue for several 
years in succession, owing to the large num.ber of pup^ 
the soil contains during the winter, after the season is over. 
Such pupae are very hard to get rid of as soil dressings 
such as lime or gas lime are of very little value. Digging 
over the soil exp>oses considerable numbers of pupae to 
the attacks of insectivorous birds, while over large areas 
deep ploughing might possibly effectively bury a large 
proportion of themi. 

The Onion Fly {Hylc7Jiia cepeto77m?) (^17) is closely 
related to the Cabbage Fly and is a common pest wherever 
onions are grown ; it is a greyish Insect very like the 



iiS 



Cabbage Fly. The Flies are common in April and ]\Iay 
laying their eggs on the necks of onions or on the leaves 
just above the soil surface. After about a week, the eggs 
hatch into larvae which becomxC fully grown in three to four 
weeks, and are then about |in. long. They turn to brown 
pupae 111 the soil, though a few m^ay remain in the bulbs, 
and the Flies commence to appear about fourteen days 
later. There are several broods in the year though the 
exact number has not been determined ; the winter is 
passed as pupee which give rise to the early Flies of the 
next year. In the earliest indications of an attack the first 
leaves become yellow and then whitish, followed by other 
leaves behaving in a similar manner. Very young plants 
are usually nearly eaten through just above the forming 
bulbs, and the larvae migrate through the soil to attack 
fresh plants. As the onions increase in size each may 
shelter a num.ber of maggots which devour the interior 
and render it rotten. As regards remedial measures 
insecticides are useless against the maggots on account of 
their burrowing habits. All infected onions should be 
pulled up and burnt. Earthing up the young plants is 
valuable as it protects the forming bulbs. Early sowing 
is also to be recommended in order to get the plants well 
started before the Flies appear ; or the seeds may be sown 
under glass February and planted out in April. Trench- 
ing and burying the soil containing the winter pup^ as m 
the case of the Celery Fly, should not be neglected. 
Various substances are also advised in order to deter the 
females from laying their eggs, and one or other of the 
following methods are useful. Watering, or better still, 
spraying the bases of the plants with an emulsion consist- 
of two to three pints of paraffin and i lb. of soft soap 
dissolved in one gallon of boiling water. To this pour 
seven to eight gallons of soft water. Add the paraffin 
vvhile the soap solution is hot and churn the mixture -very 
thoroughly by syringing it back into itself so that no free 
paraffin remains on the surface. A mixture of one bushel 
of soot to two of finely powdered lime is also recom- 
mended. In America they advise a mixture of carbolic 
acid and lime. Three pints are slaked with a gallon of 
water and a tablespoonful of crude carbolic is added after- 
wards. This should be well watered round the bases of 
the plants. 

Growers of bulbs often lose many plants from the 



iig 

Narcissus Fly [Merodon eqiiestris) (14), -which is a large 
hairy Insect marked very like a small Bumble Bee. It 
appears in spring and early summer up to July, flying in 
the sunshine over the beds. The eggs are laid on or near 
the leaf bases or on the necks of the bulbs if the latter are 
exposed. Narcissi and Hyacinths are the bulbs chiefly 
attacked. The method by which the young larvae enter 
the bulbs is doubtful; they either penetrate between the 
scales of the neck or crawl outside to the base of the 
bulb and then gnaw their way within, or possibly both 
methods may occur. When fully growm the larvae measure 
fin. long and in this stage they pass the winter, usually 
turning to the pupa in the spring, either m the soil or some- 
times in the bulbs. The life-history is said to occupy two 
years, but this is probably erroneous. The presence of 
these larv^ can be usually detected by gently pressing 
the bulbs, the infected ones being less hard, but it is often 
impossible to be quite sure without cutting open the bulb. 
Before planting, or just after lifting, all infected bulbs 
should be burnt, and any bulbs, concerning which there is 
the least suspicion, should be steeped for an hour in water 
at TIG deg. F. ; if this temperature be not exceeded no harm 
should accrue to the bulbs, while any maggots present are 
said to die afterwards as the result. Possibly a lower 
temiperature is effective and growers are advised to deter- 
mine this point for themselves. Any bulbs that fail to 
appear or undergo very little growth in spring should be 
dug up and destroyed as soon as possible. 

The House Fly {Mitsca domestica) (16) although quite 
harmless to vegetation is of great importance economically. 
Its eggs are usually laid in fermenting accumulations of 
horse manure, but may be also deposited in decaying 
vegetable refuse, ash-pit contents and other substances. 
The f empale Fly lays upwards of 120 to 1 50 eggs at a time, 
and each is capable of laying Ave to six such batches. 
The length of the complete life cycle depends very largely 
upon temperature, and during hot weather it ma\' only 
occupy three weeks from the time the eggs are laid up to 
the time when the resultant Flies emerge. Certain of the 
late autumn Flies survi^'e the winter and give rise to the 
maggots in the following spring; the Flies appear in their 
greatest profusion during x\ugust and September. The 
House Fly is injurious to man m acting as a carrier of 
disease germs, and it is specially concerned with the spread 



120 



of infantile or summer diarrhoea and typhoid fever. It is, 
therefore, of prime importance to give the Insect no oppor- 
tunities for breeding and thereby providing a check upon 
its abundance. All accumulations of manure and refuse 
should be removed at least once a fortnight, or more often 
if possible. The adoption of closed ash-bins excludes the 
access of the Flies to their contents, and are most effective 
in this respect. Accumulations of farm manure provide 
nutriment for enormous numbers of House Fly maggots. 
Experiments on a large scale are being conducted both m 
this country and America for the purpose of rendering 
manure heaps repellant to the Flies and their maggots 
and, at the same time, still retaining their valuable fer- 
tilising properties. When troublesome in houses the House 
Fly can be readily poisoned by using one teaspoonful of 
formalin added to a teacup ful of water poured into a 
soup plate. The mixture should be sweetened with a little 
sugar or should contain about 25 per cent, of milk. If 
placed at night large numbers of the Flies will partake 
of it in the early morning and are poisoned thereby. The 
mixture also has the advantage of being too weak to be 
harm.ful to human beings or domestic animals. 



Chapter 16. 



INJURIOUS ANIMALS (Continued). 

Aphides and their life-histories ; preventive and remedial 
measures. The Apple Sucker, Scale Insects. Greenhouse Pests 
and Fumigation Methods. 

The present lecture is devoted to a consideration of 
certain injurious Hemiptera. Members of this order of 
Insects are characterised by the presence of a jointed 
rostrum or beak, enclosing two pairs of stylets used for 
piercing the tissues of plants and imbibing sap therefrom. 
Nearly always four wings are present, the young re- 
semble the adults in general form, and a pupa stage is 
almost always absent, l^he family of the Aphididae is 
of great importance, including as it does the " Green Fly " 
or " Plant Lice.'' Aphides may draw the sap from all 
parts of plants, even the roots, and the injuries they 
cause are often great. They bear near the end of the 
body a pair of tubes, which secrete a substance commonly 
termed '"honey dew." Ihis accumulates on the leaves, 
blocking up their stomata, and also provides nutriment 
upon w^hich various Fungi develop. Aphides undergo a 
remarkable life cycle. In the autumn we usually find 
the fertilised winged females. The eggs laid by them 
develop the following spring into wingless females. 
These latter breed with great rapidity by parthenogenesis 
— I.e., without the agency of the males, none being 
present. Eggs are not laid but living young are brought 
forth, and this goes on for several generations until the 
.summer. Winged females then appear but there are still 
no males, and living young continue to be produced. 
Aiter a variable number of generations of this kind, 
winged males and winged females become evident, and 



121 



122 



the eggs laid by the latter give rise to a similar cycle m 
the following spring. Some Aphides are restricted to a 
particular species of plant, while others have alternate 
hosts. Thus the Hop Aphis winters on damson and flies 
to hops in the spring. Fhe Elm Aphis goes to the roots 
of Ribes, the Mealy Plum Aphis to rushes and aquatic 
grasses, and the Bean Aphis to mangolds, poppies, dock, 
etc. Some of the most destructive Aphides are the Bean 
Aphis {A-phis rumicis), Currant Aphides {Kho-palosi-phoii 
ribis Linn, and Myztis ribis Linn.), Hop and Damson 
Aphis [Phorodon hianuli), Apple Aphides {Ap/ns fo7ni 
De Geer., Afhis sorbi Kalt., and Afhis iitchii Sand.), 
Plum Aphis {^A-phis pitni Reaum.), Cabbage Aphis [Aphis 
bras suae Linn.), Turnip Aphis {Aphis rapae Curt.), Rose 
Aphides (species of Sipho?io phord"). Woolly Aphis {Schi- 
zoneura lanigerd), and others. In dealing with these pests 
it is important to remember that when using insecticides 
contact insecticides only are of use. Insecticides fre- 
quently do not destroy the eggs and, notwithstanding 
spraying, fresh broods appear from the unaffected eggs. 
When the leaves of the host plant are curled insecticides 
are of little value, as they do not reach the Aphides 
within. Frequently the undersides of leaves alone shelter 
the Aphides, and spraying must be adjusted so as to 
reach them. The earlier measures are applied after the 
appearance of Aphides, the better the chances are of 
success, before the latter become numerous. 

The Bean Aphis (17) is usually black and very con- 
spicuous; it especially attacks broad beans. It appears 
when the beans commence to bloom and attacks the heads. 
It breeds rapidly, covering the plants with a black sticky 
mass which gradually extends downwards over the stems 
and leaves. i\ simple and effective measure is to pick 
off the infested tops as soon as any i\phides are seen 
thereon, and drop them straight away into a pail of lime. 
In bad infestations spring spraying with a knapsack 
sprayer is necessary. Two applications are desirable and 
are usually successful. The Cabbage Aphis (17) may also 
infest swedes and turnips. it usually appears about 
May, but evidences of injury are not generally noticeable 
until June, when the leaves begin to show blister-like 
areas on the upper surface, while the Aphides are to be 
found in the corresponding hollows on the undersides of 
the leaves. The leaves become yellow and discoloured, 



and in late summer the plants may swarm with the 
Insects. In the early stages of attack benefit is derived 
by cutting- off the blistered or yellowish leaves and 
destroying them. Later on dusting with soot is worthy 
of a trial, and spraying is practicable on a small scale in 
garden plots. All methods of cultivation tending to 
]3roduce vigorous growth are serviceable, and copious 
watering in dry weather, v/hich favours Aphid multiplica- 
tion, is of great value. Aphis fomi and A, sorbi (4) which 
attack the apple, puncture the leaves and cause them to 
curl and becom.e discoloured, while A. fiJchii attacks the 
blossoms and buds, but does not usually cause leaf 
curling. For all three species it is important to spray 
about the middle of i\pril, when the eggs have hatched 
and the young are most vulnerable. The Rose z\phides 
are familiar to every gardener, and there are three species 
commonly met with. Si-phono fhora dirhoda Wlk. is 
stated to migrate to grasses, Polygonum and wheat, 
5. rosae Reau. to teazles, while S. rosantm Wlk. appears 
to have no alternate host. If only the latter species be 
present one or two early sprayings are sufficient, but w^ith 
the other two species additional applications are some- 
times necessary owing to fresh infections from their 
other plant hosts. Paraffin should never be used on rose 
trees, and only ilb. of soft soap should be mixed to 10 
gallons of water m the quassia w^ash referred to further 
on. When only affecting a few twigs here and there. 
Rose Aphides can be readily killed by momentary immer- 
sion in a vessel of water just too hot to keep the hand 
in, without injury to the plant (10). Of the Currant 
Aphides (4), R. ribis produces reddish blister-like galls 
on the surface of the leaves, while M. ribis causes the 
leaves to curl up especially those on the terminal shoots. 
Both species as they become numerous are difficult to deal 
with, as they are protected in the hollows of the blisters 
on the undersides of the leaves, or within the curled up 
leaves. One of the eiTects of their presence is the fre- 
quent falling of the fruit before maturity. Both species 
deposit their eggs under the broken rind or upon it, 
chiefly on tw4gs of the previous year's growth. Both 
currants and gooseberries are attacked, and M. ribis 
especially frequents black currants. Early spraying 111 
April is the best measure, and care must be exercised m 
order that the fluid reaches the undersides of the leaves. 



124 



The Plum Aphis (4) appears before the buds open, and 
the parent form may be found in March. They are dull 
fat purple Insects, while the young are olive green at 
first, becoming purple later on. The latter attack the 
young unfolding leaves and soon cause them to curl, 
not only those of plum and damson, but also allied fruit 
trees. Spraying should take place as soon as the Aphides 
are noticed and before the buds are open, if possible, as 
the Insects are then most readily destroyed. Further 
spraying late m September and in October helps to kill 
off the egg-laymg females. 

The \\'oolly Aphis, or American Blight (4) is a univer- 
sal pest of apple in this country. By constant sucking of 
the sap it lessciis the vitality of the trees; their punctures 
in the bark and young wood cause abnormal growths of 
soft tissue which form characteristic rounded swellings. 
Later on these swellings split and from them arise large 
rugose deformities often ascribed to canker." These 
wounds further predispose the trees to the attacks of fun- 
gus enemies. Under ground this Insect further causes 
gall-like swellings on the roots. As a result of Wooly 
Aphis attacks, young trees may die, stunted trees often 
result, and the fruit is deficient and of poor quality. The 
parent wingless Aphides are reddish or purplish-bro\\'n, 
and are invested with a white wooly substance. Living 
young are produced and become similarly invested with 
this white material, forming conspicuous objects on the 
branches of the trees. The wingless egg-laying females 
and vringed males occur m autumn. Each female is very 
small and lays a single egg near the foot of the tree, 
and the egg hatches the following spring. Winged 
parthenogenetic females appear to be rare and are seldom 
met with. In the winter the Insect lives in the adult 
state on the bark, or in the roots below ground, and also 
in the egg stage. The possibility of resistent varieties 
of apple is worth}' of attention, especially as in Australia 
the roots of apples grafted on to the Northern Spy and 
Majetin (an English apple) are said to be proof against 
this Insect. Spraying with soft soap and quassia is an 
efficient summer treatment, but the solution must not be 
spared, and force is necessary or the wooly covering of 
the Aphide will not be wetted. Ihe only vvay of getting 
rid of the root forms is by injecting bisulphide of carbon. 
For an average-sized tree four i oz. injections into the 



125 



soil 2 feet away from the trunk are snflicient. The fluid 
must not reach any of the larger roots and, moreover, it 
IS very poisonous and highly inflammable. If summer 
spraying is not sufficient, a winter wash, as detailed m 
Leaflet 34 of the Board of Agriculture, is advisable. 
Banding the tree trunks in early spring is said to give 
encouraging results, and several cases have been reported 
of large numbers of Aphides migrating from the roots, 
being trapped on these bands as they were ascending the 
trunks. " Tree tanglefoot'' is best for this purpose. 

As regards preventive and remedial measures (4, 1 5) 
against Aphides there are three seasons of application, 
(a) In autumn to kill oft the egg-laying females. The 
leaves are then of much less value, and it matters very 
little if they are injured. Thorough spraying vrith a 
mixture of i pint paraffin and iMbs. soft soap added 
to ic galls, of soft water, which should be made up in 
the manner suggested in the previous lecture for dealing 
with the Onion Fly. In the case of Currant Aphides 
heavy pruning is valuable, as the eggs are present in 
large numbers on the shoots. All prunings should be 
taken away and burnt, [h'j Winter measures: these con- 
sist of using sprays, which have the eftect of sealing up 
the eggs with a coating through which the young Insect 
is unable to make its way. A good mixture recommended 
by Theobald consists of i cwt. of fresh lime, which 
should be gradually slaked, and mixed with 100 galls, 
of water m which 3olbs. of salt have been dissolved. The 
addition of Slbs. of water glass is stated to be an advan- 
tage, though not essential. Failure in obtaining satis- 
factory results are usually to be traced to want of care 
in starting with freshly-burnt lime, or in slaking this. It 
is best used as a late winter wash, as its efl"ects wear oft' 
owing to weather action if it remains on the trees all 
through the winter. ihis wash is useful m sealing up 
the eggs of the Apple and Plum Aphides, and may also 
be used for the same purpose against the Currant Aphides 
if pruning has not been done, ^c) Spring spraying, which 
should be done as early as possible. A useful mixture 
for most Aphides is made up by boiling for 2 hours lib. 
of quassia chips (which must be quite fresh) in just 
sufficient water to keep liquid. This solution should" loe 
strained, and then well mixed with 10 galls, of warm 
water, m which lib. of soft soap has been previously clis- 



126 



solved. V\ ashes containing paraiiin are liable to injure 
the delicate spring foliage, and if adopted should be 
used _ m weak strength. When the leaves commence to 
curl it is waste of most insecticides to use them; nicotine 
and soft soap compounds are the only ones which offer 
any prospect of partial success. Summer spraying is not 
to be recommended except m the case of the Woolv Aphis. 
^ Another family of flemiptera, viz., the Psyllidae, or 
J umpers," includes the well-known Apple Sucker 
[Fsylid jjialt] '4 , which is a not distaait relation of the 
Froghoppers, or ''Cuckoo Spir" Insects. The adult Apple 
Suckers are small, greenish- }-ellow, four-wmged Insects 
about -in. long. They are to be found flying and leaping 
about apple leaves from jlay until the autunm. They lay 
their eggs from late September until early 111 November, 
usually on the bark of one-year shoots below buds or 
around leaf scars. The eggs are orange, darkening to 
orange-red, and hatch m April. The young larv^ are 
very minute, flattened, dirty-}'ellow Insects with red 
eyes, and they secrete a waxy substance from the hind end 
of the body. As soon as the buds open they congregate 
withm, while the older larvae and n}'mphs are to be found 
on lire undersides of the leaves. Damage is caused by 
the larv^ and nymphs piercing the young leaves, which 
become brown as if frost-bitten, and wither. In this way 
floral and leaf buds are destro}'ecl wholesale. The 
adults cause a relatively small amount of injury. Un- 
doubtedly the most vulnerable period m the life-history 
of the Insect is when the young larvae emerge from the 
egg ; for various reasons, however, spraying at this time 
presents difficulties. Owing to the waxy substance which 
the larvee exude, sprays should contain a wax solvent 
which, however, is liable to damage the developing 
leaves. Furthermore, the larvae- very soon enter the buds, 
and then spraying is of little value. The larvse emerge 
from the eggs during several weeks, and the time appears 
to vary in different kinds of apple; for this reason 
several sprayings are necessary. The best period for 
dealing with this species is apparently February and 
IMarch, 2-3 weeks before the huds open. A wash of 
lime and salt recommended by Theobald appears to be 
an eft^ective measure. It should be used on dr}^ days, and 
is made by taking i-i^ cwts. of best quality lime, slaking 
it gradually, and mixing it with icq galls, of water in 



12; 



wliich 30-4olbs. o£ salt have been dissolved. The mixture 
should then be strained through sacking or other material 
before being used. Lime washes are useful in other 
ways and beneficial to the trees. This mixture coats the 
eggs and prevents them from hatching, and also seals 
up the buds protecting them from any larvae that may be 
hatched. In the autumn, spraying with paraffin is also 
valuable — it should be done as soon as the fruit have 
been gathered, so as to kill the females before the eggs 
have been laid. Paraffin 4pts., soft soap lilbs., and 
water lO galls., forms a suitable mixture, but a stronger 
proportion of paraffin can be used at this time of the year 
if desirable. Heavy spraying is necessary, not only on 
the leaves but also on the clouds of Apple Suckers which 
are disturbed and take to the wmg. 

The family of the Cocci da^., or Scale Insects (4), in- 
clude some highly injurious members. The females are 
degenerate, and spend their life hidden beneath a scale- 
like covering formed by the cast skins of the larvae, cuti- 
cular secretions, and other means. The males live under 
smaller but similar scales, and when mature issue as 
minute winged Insects. The Mussel Scale {^Lefidosafhes 
ulmi = Mytilaspis foinoruni) is the commonest and best 
known species, and is an abundant pest of apple trees. 
During the spring and summer it sucks the sap, and 
passes the winter in the eggs which are hidden beneath 
the parent female's scaley covering which still remains. 
One or other of the Woburn washes (4, 1 5) have given 
good results in destroying the eggs of this Insect. They 
can be used any time during the winter, and the lob 
formula given by Pickering and iheobald (15) is as good 
as any. The Brown Scale {Lecanium fersicce) often 
attacks currants and gooseberry, and the Woolly Currant 
Scale' {Pidvinaria vitis) both currants and vines. For 
use against these two Scale Insects the wash lob referred 
to above, is generally recommended and should be well 
sprayed over the bushes during January. With regard 
to Scale Insects on vines different treatment is necessary, 
and the same applies to Mealy Bug and to the Green- 
house White Fly. The latter belongs to an allied family 
of the Hemiptera, viz., the Aleurodida^, and is a minute 
moth-like white Insect with four wings. Its larvae are 
green and resemble young Scale Insects, and are destruc- 
tive to tomatoes and other greenhouse plants. For 



128 



Insects in greenhouses which attack the leaves of plants, 
and cio not live m the soil, fumigation v/ith hydroc}'anic 
acid gas is the best general remedy. Ir is made up as 
follows: — 2 ozs. cyanide of potassium, 4 ozs. sulphuric 
acid, and 8 ozs. of water. iiLStimate cubic space of green- 
house and use this preparation for each 1,000 cubic feet 
of space, doubling the quantity for 2,000 cubic feet, and 
so on. First close up all windows and doors and any 
holes should be securely hlled up. One window should 
alone be left open for purpose of introducing the mix- 
ture, which is extremely poisonous to man and all forms 
of animal life, but does not injure plants. Use at ditsk 
when the plants are dry, and the temperature of the 
greenhouse should not exceed 60 F . — the best temperature 
IS 50 F. Do not use when there is a bright light 'clay- 
light). Pour the water into a jar hrst, and then add the 
acid slov%'ly. V\'rap the cyanide m a piece of blotting- 
paper and drop into the jar with a suitable instrument 
from outside through the window of the greenhouse. 
Then close the window and leave for at least i hour. 
Afterwards open all doors and windows from the outside, 
but do not enter the greenhouse until another hour has 
elapsed. Cost about 6d. 



Chapter 17. 



BENEFICIAL ANIMALS, &c. 

Birds, The Testacella Slug. Earthworms, Centipedes. Bene- 
ficial Insects. The Isle of Wight Bee Disease. Literature bearing 
upon Economic Zoology, 

Beneficial animals are on the whole less widely known 
than injurious species ancl, unlike the latter, they should 
be encourag-ed so far as ma\' be possible and under no 
circumstances be destroyed. BIRDS (i, 2) occupy a very 
high place as benefactors of the farmer and horti- 
culturalist. There are a number of kinds which cause 
little harm, and are 111 many cases, direct!}- beneficial. 
Among_ these may be cited the Fieldfare, Hedge Spar- 
row, \\Tens, Long-tailed and Coal Tits, Wagtails, 
Pipits, Swallow, ?vlartins. Swift Cuckoo, Plover, and 
many others. Tits, for instance, are particularly partial 
to Scale Insects, as well as Aphides and other Insects, 
i he Willow Wren devours large numbers of various In- 
sects, and Xewstead records one whose crop was filled 
with larvbe of the Winter ]\Ioth, another with Aphides, 
and three other individuals contained large numbers of 
t ly Alaggots. Pipits, the Cuckoo, and Swift are also 
prominent devourers of Insect life. Occasionally, how- 
ever, one or other species may be observed devouring 
fruit, the ^Iistle Thrush, for instance, but I believe in 
such cases the small amount of harm they cause is neglig- 
able compared with the benefit they confer. In this 
country much could be done along Continental lines to- 
w^ards encouraging beneficial Birds, especially by means 
of nesting-boxeS; which help to ensure their presence in 



130 



desired areas (ig). I'he Department of Economic 
Zoology in this University has achieved some good re- 
sults in the larch plantations of the Manchester Corpora- 
tion catchment area around Lake Thirlmere. Here the 
Larch Saw-fly is most destructive, and by fastening on 
the trees large numbers of nesting-lDoxes suitable for Tits, 
which prey on the larvae of this Insect, direct benefit has 
resulted. 

Among Molluscs the carnivorous Slug Testacella 
alone is valuable. It is a dirty white or yellowish form 
with a small shell situated at the hinder end of the body. 
In injurious Sings the remams of the shell is always very 
far forwards and m close relation to the respiratory 
pore. It is local in this district, but has occurred in 
several localities. Testacella feeds upon other Slugs, 
Worms, and dead animal matter, and causes no harm to 
vegetation., 

Earthworms are true segmented worms and differ 
from Eelworms. In the course of burrowing Earth- 
worms let in moisture and air, the subsoil becomes 
loosened, and direct benefit therefrom is derived. Large 
quantities of earth are swallowed by them, which they 
pass out of their bodies m the form of worm casts," 
commonly seen on lawns and flower -beds. In this 
manner fresh soil is constantly being brought to the sur- 
face, and at the same time Earthworms draw numerous 
leaves and other kinds of vegetation into their burrows 
which they consume in appreciable quantities. Humus is 
partly due to the activities of Earthworms — the bringing 
of soil to the surface and the burying of vegetable 
material is an important factor in the humus formation, 
which adds to the general fertility of the land. Darwin 
calculated that as much as ten tons of soil per annum 
passes through the bodies of Earthworms and is brought 
by them to the surface, over each acre of good land. 

AllLLIPEDES (7) belong to the class of Myriapoda 
which are more closely related to the Insecta than to any 
other group. Like the Insects, Myriapods are provided 
with a single pair of feelers or antennae, but they always 
possess more than six pairs of legs, usually ^ a large 
number, and never acquire wings. The two main groups 
of the ]^Iyriapods are the Centipedes or Chilopoda and 
the Millipedes or Diplopoda. The former, which are 
beneficial rather than injurious, have a somewhat flattened 



131 

body and a single pair of legs to each segment. So far 
as known they feed upon small worms, slugs, insect 
larvae, etc., and also upon dead animal matter. The Milli- 
pedes may be readily recognised by the possession of two 
pairs of legs to each body segment. They occasionally 
cause injury to potatoes and other root crops, and are 
often known as " false wireworms.'' 

Among Insects ^ro) many kinds are beneficial and 
may be divided into two groups, (ij Predaceous Insects 
which attack other Insects directly and devour them, and 
immediate benefit is derived from their action. Among 
these may be mentioned Ground Beetles, Lady-Birds and 
their larvse, the larvae of Hoverer Flies, Wasps, Robber 
Flies, and others. (2) Parasitic Insects which deposit 
their eggs in the bodies of other Insects or in their 
immediate neighbourhood. They pass the greater part 
of their life wdthin their hosts, whose death they sooner 
or later bring about. Parasitic Insects amount to tens of 
thousands of species and constitute Nature's most effec- 
tive method of control over the excessive multiplication 
of Insect life. They are almost exclusively confined to 
the orders Hymenoptera and Diptera. One or tw^o of the 
most important families of beneficial Insects may be 
briefly mentioned. 

Ground Beetles, or Carabida:^, form a very exten- 
sive family, comprising a large number of British 
species. They can be recognised by their hard convex 
bodies, long thin legs and slender feelers, and their very 
active running habits. The majority are beneficial, de- 
vouring other Insects, Molluscs, and dead animal matter. 
Some few, however, are knowm to be injurious, but it is 
a safe rule never to destroy these Insects when seen, with 
the exception of those frequenting strawberry beds, w^hich 
usually pertain to a harmful species. The larvas of 
Ground Beetles have very similar feeding habits to the 
adults. They are elongate fleshy creatures, with _ a 
definite hard, brown head and first segment, three pairs 
of legs and two horn- like processes, either long or short, 
at the hinder^ end of the body. They frequent^ the soil, 
can run freely, and their pupae are found buried some 
niches below the surface. Lady BIRDS are beetles be- 
longing to the family of the Coccinellidae. We have a 
number of species in this country, and one of the com- 
monest and best known is the Seven-Spot Lady Bird 



13^ 



[CGccineila scfteinpunctata). Both as larvsr and adults 
Lady Birds devotir great numbers of Aphides and Scale 
Insects , and for this reason they should never be 
destroyed. The females deposit their eggs as a rule on 
Aphid infested plants so that their larvae may not have 
tar to vs-ancier for their food supply. The Beetles are 
all very similar m shape and are mostly black and red, 
or black and yellow m colour. The}' hibernate during 
the winter beneath bark of trees, under rubbish and m 
outhouses, etc. In the follovdng spring they lay their 
cream-coloured eggs closely packed together in groups. 
The larvae are black or leaden-coloured, marked as a rule 
with }'ellow or orange. They cravrl freely about the 
plants and consinne great numbers of Aphides and other 
Insects. The pupar are attached to the upper or under- 
sides of the leaves and are broad black objects marked 
with cream-colour or \-ellow. I'he adult Lady Birds 
appear earh* m summer and are common objects of the 
field and garden throughout the season. It is note- 
worthy that the destructive Scale Insect Iccrya furchasi 
which devastated the orange groves of California 
has been almost entirely destroyed and checked by the 
importation into America of an Australian Lady Bird 
l\ oiUiS c jrdinalis. The Scale Insect has thus remained 
permanently controlled, and the y o'iiiis Beetle is now a 
regular resident in California. The orange Scale Insect 
has been controlled by a similar measure m Florida, Xew 
Zealand, Portugal, Ca|_>e Colon}', F ormosa, iigypt, h ranee, 
and other countries. HOVERER FLIES belong to the family 
of the S}U'phidce. They are often brilliantly coloured, 
being black with yello'A bands, and have the appearance 
of small Wasps. The}" hover m the air, remaining 
stationar}', except for their vibrating wings, over one spot 
for several minutes and then, darting away sudclenl}', 
hover again over a fresh spot. The}' onl}' liy m stmshine, 
and rest on leaves and flowers m dull and wet weather; 
they feed mainly upon nectar. 3.Iost species of Hoverer 
Flies lay their eggs among colonies of Aphides, and 
their maggot-like larv^ on emerging feed voraciously 
upon the latter. When fulrv fed the pupae are to be 
found enclosed m membraneotis puparium on the leaves 
and stomas of plants close to vmere the larvae lived. The 
o:reater number of the members of this family are, there- 
fore, beneficial Insects. 



133 



Ichneumon Flies and their allies are parasitic Insects 
belonging to the order Hymenoptera, and are related to 
the Bees and Wasps. They can be recognised by the 
presence of four transparent wings^ and by the posses- 
sion in the female of a long needle-like ovipositor 
or egg-laymg instrument. Almost every species of In- 
sect IS preyed upon by some other species of Ichneumon 
Fly or its ally. By m.eans of their ovipositor these 
Insects pierce the skin of other Insects and lay their eggs 
withm the body of the latter. The host Insects are not 
killed by this operation, but continue feeding. The 
larva^ of the parasite hatch out m due course and 
gradually devour the blood and tissues of the host, avoid- 
ing, however, the vital organs until the very last. The 
parasite turns to the pup^e either within or outside the 
host Insect, and just about the same time the latter dies. 
Some Insects are greatly destroyed b}^ parasites and thus 
kept in check naturally — the commxon Hawthorn Scale 
and the Gypsy Moth are familiar examples. In America, 
however, the Gypsy Moth has got introduced artificially, 
and has now spread with alarming rapidity and causes 
millions of dollars damage annually. Unfortunately the 
natural enemies of this ^vloth were not imported, and the 
U.S. Department of Agriculture recognise that the only 
way of ridding the country of this Insect is by importing 
its natural enemies or parasites from Europe. This work 
IS now steadily going ahead on a very large scale, but 
it IS too early to know the final result. In Italy mulberry 
cultivation has been threatened by a Scale Insect. Diasfis 
pejttagona. In i8gi the pest a-ssumed such serious pro- 
portions that the Italian Government passed a legislative 
micasure compelling m.ulberry cultivators to use all avail- 
able means for coping with it. This, how^ever, proved of 
little efficacy, and it was not until Professor Berlese, of 
Florence, introduced great numbers of a minute parasite 
from America into Italy that much headway was made. 
From all later reports it appears that this parasite has 
acclimatised itself to Italy and is proving of great value 
m destroying the Mulberry Scale. 

Of all beneficial Insects the Hive Bee is the most use- 
ful species. It is desirable here to make some reference 
to the Isle of Wight Disease (i8), which is causing much 
m.ortality among apiaries all over the country. The 
epidemic is due to a minute Protozoan organism iNosenui 



^34 



afis), and the disease is spread by the distribution of its 
spores among unaffected bees. The disease is primarily 
one of the digestive system, and aftected Bees are, as a 
rule, unable to fly more than a few yards without alight- 
ing. As the disease progresses the Bees can only fly a 
few feet from the hive, and then drop and crawl aimlessly 
over the ground. They may often be seen crawling up 
grass stems or up the supports of the hive. Diseased 
Bees frequently lose their power of flight, their abdomens 
become greatly distended, and often the wings are out 
of joint,'' the hind wings protruding upwards and outside 
the anterior pair. In bad attacks large numbers of 
diseased Bees are to be found clustered together within 
the hive, or on the alighting board and ground. Some- 
times the symptoms resemble those of " Bee paralysis or 
of dysentry," but nevertheless the disease is quite dis- 
tinct from either of these complaints. The spores of the 
disease are spread in various ways; thus water near the 
hives becomes infected with Bee excrement, containing 
the spores,, which are liable to be imbibed by the next 
Bee which visits the same spot. Honey, pollen and wax 
also become infected with the spores m a similar manner, 
and are very fertile sources for spreading the disease. 
Infection from one hive or apiary to another can be 
effected by the sale of diseased swarms, by the robbing 
of a diseased colony by healthy Bees, and by swarms 
occupying hives which w^ere formerl}/ infected. Bad 
weather also encourages the spread of the complaint, as 
the bees do not then take cleansing flights. There is 
evidence to indicate that partial immunity of stocks 
happens. Such stocks, however, might be hard to recog- 
nise, and at the same time would act as sources of infec- 
tion for susceptable colonies. As regards measures 
against the disease, healthy stocks should be remo\'ed 
from the neighbourhood of diseased hives. Clean water 
should be readily accessible, changed daily and protected 
from contamination. The usual drinking places should, 
if possible, be done away with. All dead Bees should 
be burnt and diseased colonies, including the queens, 
destroyed. The ground around the hives should be dug 
over and treated with quicklime. Infected hives and all 
utensils should be charred inside and out with a painter's 
lamp. In place of charring a very thorough application 
of formalin or carbolic acid may be used, and the hives 



135 



afterwards properly aired m strong sunlight. Xo cure 
for the disease has so far been discovered. The problem 
of hereditary infection is of great importance, but 1 am 
not aware that any evidence thereon is yet forthcoming. 
If the queen is capable of transmuting the Xoseyna 
parasite to the eggs, the young brood would thus be born 
infected, and the disease be passed from one generation 
to another, as happens in the case of the A osema which 
causes Pebrine m silkworms. Investigations along these 
lines are greatly needed. 



LITERATURE BEARIXG UPOX ECOXOIMIC ZOOLOGY. 

(1) Xewstead. R. — The Rood of Some British Birds. Sii^^le- 

rnejit to Jciirnal Bd. of Agriculture . vol. xv. iqoS U^.). 

(2) CoLLlXGE, X\ . E. — The Food of Some British AVild Birds. 

Dulau & Co.. 1Q13 U/-)- 
13) Leigh, H. S. — Interim Report on The Feeding Habits of the 

Rook. London: 'His Alajesty's Stationery Onice. igif. 
U) Theob.VLD. F. V. — Insect Pests of Fruit. Published by the 

.Vuthor. "Wye Court. V\"ye, igoQ (A'l is. ?). 

(5) Russell, E. J., and Peieikrbridge, F. R. — Partial Steriliza- 

tion of Soil for Greenhouse V^ork. — Journal Bd. of Agri- 
culture, vol. xviii. iQi2_. p. 8cQ. 

(6) Theobald. F. — Injurious and Beneficial Slugs and Snails. 

Journal Bd. of A^griculture , vol. xi. IQ05, pp. 594 and 650. 

(7) Theoe.VLD. F. v. — Second Report on Economic Zoology. 

London : British ZMuseum '.Xatural History-), 1Q04. 

(8) CoLLiXGE. \V. E. — The Economic Importance of AVood-Lice. 

Journal Bd. of A^griculture , vol. xxi. 1Q14. p. 206. 
(q) Carfexter. G. H. — Insects, their Structure and Life. Dent 
and Co. (4/6). 

(10) ]\Lall_. L. C. — Injurious and Useful Insects. Bell & Sons. 
(3/6). 

mi) Wllllims. C. B. — The Pea Thrips. A^nnals of Af plied 
Biology, vol. i, IQ14-15, p. 222. 

(12) Leaflet, Xo. 10. AVire V\"orms). Bd. of Agriculture. 

(Gratis on application.) 

(13) Theobald. F. — First Report on Economic Zoology. 

British Museum (Xatural History). ig:3. 



136 



(14) Narcissus Flies. Journal Bd. of AgriculUire , vol. xvi, 1Q14, 

p. 136. 

(15) Pickering, S., and Theobald, F. V.— Fruit Trees and their 

Enemies with a Spraying Calendar. Simpkin, Marshall, 
Hamilton, Kent & Co., 1Q08 (1/6). 

(16) Hewitt, C. G. — House Flies and how they Spread Disease. 

Cambridge University Press, 1Q12 (i/-). 

(17) Ormerod, E. x\. — Alanual of Injurious Insects. Simpkin, 

Marshall, Hamilton, Kent & Co. (5/-). 

(18) Imms, a. D.— The Isle of Wight Bee Disease. Journal Roy. 

Agricultural Society, vol. Ixxv, 1914, p. 62. 
(iq) Hiesemann, M. — How to Attract and Protect Wild Birds. 

Witherby & Co., 1Q12 ii/6). 
(Nos. Q and 10 are good elementary works on Insects; No. 4 
is the best book of reference on fruit pests, while No. 1 5 is an 
elementar}' outline of the same subject : Nos. 7 and 13 are reports 
on various species of injurious Insects and other animals; No. 17 
is a general book on pests of fruit and vegetables and, although 
out of date, contains useful information ; Nos. i and 2 deal with 
the economic status of Birds, and No. ig with the encouragement 
and protection of the beneficial species. The remainder of the 
references deal for the most part with special subjects referred 
to in these lectures.) 



INDEX. 



PAGE 

Abraxas grossulariata 108 

Acari 103 

Acid manures — effect on fungal 

growth 70 

Acid soil 11 

Acidia heraclei 115 

Aecidiospores on barberr}^ 92 

Agriolimax agrestis 102 

Aieurodida3 127 

x\merican Blight 124 

American felt-paper discs for 

Cabbage Root Fly 117 

American gooseberry mildew ... 88 

Anisopteryx aescularia '. 109 

Annuals 2 

Antennae 107 

Aphides 121 

insect enemies of 132 

Aphides, spraying methods 

against 122 

Aijhis brassicae 122, 123 

fifchii 122, 123 

2Jomi 122, 123 

pruni 122, 123 

rapae 122, 123 

rumicis 122, 123 

sorbi 122, 123 

Apple Aphides 122, 123, 124, 125 

Apple, insect enemies of 108, 109 

122, 124, 126, 127 

Apple, self sterile varieties 38 

Apple sucker 126 

Avion ater 102 

Arsenate of lead ... 108, 109, 110, 111 

Arsenate paste 108 

Artificial selection 44 

Ascent of sap 8 

xysters — wilt or Black-Leg disease... 81 
Australia, wheat rusts in 93 



Bacillus solanacearum 84 

Bacteria, nitrifying 12 

Bacteriosis of tomato 84 

Bacterised peat 13 

Banks, Sir Joseph 90 

Barberry, aecidiospores on 92 

Barberry and wheat rust 92 

Bary de 92 

Bean Aphis ....122 



PAGE 

Bee Hive 133-4 

Diseases of 134 

Beetles 112, 131, 132 

Begonia 32 

Beneficial animals 129 et seq. 

Biennials 2 

Birds in relation to other animals 

98, 99, 112, 114, 117, 129 

Black Currant Gall Mite 103 

Black currants, insect enemies of 

103, 123 

"Black-Leg " of Asters 81, 83 

Black Scab of potatoes 70 

Black Slug 102 

Black Striped Slug 102 

Black stripe of tomatoes 84 

Blackbird 98, 103 

Breeding of pLints 10 

Blister rust of pine 93 

Blood-sucking Flies 113 

Blue Tit 98 

Biffen, Prof., and disease resis- 
tance 93 

''Big bud" ; 103 

Bone Meal 14 

Books dealing with Economic 

Zoology 135 

Bordeaux mixture as a Fungi- 
cide 78, 84 

Bottomley, Professor 13 

Bracket fungi 61 

Breathing of plants 7 

of roots 7 

of germinating seeds ... 43 

pores of shoots 7 

Brown Scale 127 

Bud variation 26 

Budding 32 

Bulb 28 

Bulb, ripening of 28 

Bulbs, insects attacking 119 

Bulbil 29 

Burning sulphur as a disinfectant 86 

Burning of refuse 69. 80 

Butterflies 107 

Cabbage Aphis 122 

Cabbage Butterflies 107 

Cabbages and cauliflowers " Club 
Root" 65 



138 



PAGE 

Cabbages, insect enemies of... 107. 

lie: 122 

Cabbage Eoot Flv 166 

Cacti 23 

California, beneficial insects in ... 132 

Carabidae 131 

Callus 30, 53 

Carbolic Acid, remedy for Eel- 
worms 100 

Carbolic Acid, for sterilisation of 

hives ■ 134 

Carbolic Acid and Lime, remedv 

for Onion Fly ' ^ 118 

Carbon Bisnlpliide,. remt^dy for 

Leather Jackets 114 

Carbon Bisulphide, remedv for 
Eoot Form of Woolly Aphis ... 124 

Carbon nutrition 20 

Carnation, layering of 30 

Carnivorous Slug 130 

C cLi-'pocaijsa pornoncUa 108 

Cauliflowers, insect ti-nt^miH.- of ... 116 

Celery Fly 115 

(""ells, diseased by fingei'S-and-tots 6/ 

( flo.<i(i rrisfate 50 

(' ( rfo;-<ijora rndon'K 85 

Thaffinch 98, 99 

Cheimafohia hrvmata 103 

Cherry, insect enemies of 108 

Cherrv. pruning of 35 

Chilopoda 130 

ChortophUa Ijr'issimf 116 

Chrysalis , 105 

C]a(lo.^po/-iv in Tvirjiii, on tomato .... 83 

Clav soils 10 

Click beetles 112 

Climbers 18 

Club Eoot — precautions and re- 
medies 69 

Coal Tit 129 

(' occindla seijtcin-pu/ncfafa 162 

Codling Moth 109 

Coleoptera 112. 131. 132 

Conidia of cercosporci meloiiis 85 

Conidia of Thyfophihora 74 

Contact, sensitiveness to 6 

Copper sulphate 84 

. Corm 29 

Crab apple, use for pollination ... 39 

Crane flies' 113 

Crested varieties 49 

Crocus 29 

Cross pollination, effect of 37 

Cruciferae — Ckib Eoot disease 65, 66 

Cuckoo 129 

'•'Cuckoo Spit', insects 126 

Cucumbers attacked by Eelworms 100 
Cucumber, disease resistant varie- 
ties 86. 87 

Cucumber, leaf disease 84. 85 

Currant Aphides 122, 123. 125 



PAGE 

Currants, insect enemies of ... 108, 

110, 122. 123, 127 

Currant Moth 108 

Currants, pruning of 36 

Currant Scale Insects 127 

Cut-leaved varieties 45 

Cuttings 30 31 

"'Daddy Long Legs" 113 

Dahlias attacked by Earwigs 107 

''Damping off" disease 43, 61 

"Damping off" methods of pre- 

A'ention 64 

" Damping off — symptoms 61 

Damson Aphides 122 

Darwin. Charles 37. 44 

Darvrin quoted 130 

Desert plants 24 

Destruction of diseased plants 69, 

80. 88 

De Vries" mutation theory 45 

Diaspis perifagona 133 

DiiDlopoda 130 

])i[Ao.<i.< pyrxvora 114 

Diptcra 113, 132 

Discs, felt-paper for Cabbage Eoot 

Flv 117 

Disea.-e resistance 79. 86, 87 

Disease resistant varieties of cu- 
cumber 86. 87 

Diseases caused by fungi 57 

Diseases of Bees 134 

Diseases of Silkworms 135 

Diseases nf Man caused bv Insects 

' 113, 119. 120 

Dominant characters 46 

Double flowers 51 

Double-fruited orange 52 

Drainage of soil 7. 70 

" Dysentery in Bees 134 

Earthworms 130 

Earwig 105 

Eelworms 99 

Elateridae 112 

Elm Aphis 121 

Environment, effect of 24 

Epidemics of late blight origin ... 78 

Eriophyes rihis 103 

Eriophyidae 103 

Erysiphaceae 87 

Ether vapour for forcing 36 

Evening x^rimrose 45 

Eye of potato 27 

" False Wireworms " 131 

Farmyard manure 11 

Fasciation 49 



^39 



PAGE 

Fertilisation 37 

Field Fare 129 

Flies 113 

Hoverer 132 

Flowers, production of 34- 

Flower, structure of 36 

Fluctuations 45 

Fog, effect on vegetation 55 

Food materials in turnip 67 

Forficula auricularia 106 

Forcing 35 

Formalin, remedy for house flies 120 
Formalin as soil disinfectant ... 83, 86 

Forsythia, pruning of 35 

''Frog hoppers" 126 

Frost, effect of 53 

Fruits, ripening of 40 

Fumigation Methods 128 

Fungi as parasites 60 

Fungi as saprophytes 60 

Fungi, nutrition of 60 

Fusarium 82, 83 

Fusafnum lycopersici 84 

Fusarium Solani 76 



Gall Mites ' 103 

Garden Snail 101 

Gas Lime for Eelworms 100 

for Wireworms 112 

for Leather Jackets 114 

for Cabbage Root Fly ... 117 

Germination 42 

Gladiolus 29 

Gooseberry, American mildew ... 88 

Gooseberry, layering of 29 

Gooseberry, rust on , 93 

Gooseberry Sawfly 110 

Gooseberries, insect enemies of 

108. 110, 127 

Grafting \ 32 

Grass, insect enemies of 114 

Grease banding 108 

Great Tit 98 

Green Fly 121 

Greenfinch 98 

Greenhouse Pests 101, 127 

Greenhouse White Flv 127 

Grey Slug • '. 102 

Groundsel 2 

Gymnosporangium 93 

Gypsy moth, methods of control 
in America 133 



Hand picking for "big bud" ... 104 
Hand picking for cabbage cater- 
pillars' 107 

Hand picking for currant cater- 
pillars 108 



Hand picking for gooseberry saw- 
fly Ill 

Hand picking for maggots of 

celery fly ne^ 

Hand picking for bean Aphis ... 122 

Hardening off 24 

Healing of wounds 52 

Hedge Sparrow 129 

Helicidae 101 

Helix aspersa 101 

remoralis 101 

rufescens 101 

Hemiptera 121, 126, 127 

Heterodera radicicola 100 

Hive, Bee 133-34 

Hoeing, importance of 12 

Hollyhock rust 94, 95 

Honey agaric 61 

Hop Aphis 122 

Hot-water bath for forcing 36 

Hot Water, remedy for Narcissus 

flies 119 

Hot Water, remedv for Eose 

Aphis !^ 123 

House Fly 119 

House Flv as a disease carrier 

119, 120 

House Sparrow^ 98 

Hoverer Flies 132 

Humus 13, 130 

Hybrids 46 

Hydrocyanic Acid Gas 128 

Hydro-oxide of Calcium for Slugs 102 

Hylemia cepetorum 117 

Hjanenoptera 110, 133 



Icerya purchasi 132 

Ichneumon Flies 133 

Infection of asters 82 

Insect pollination 36 

Insects 105 

Methods of feeding 106 

Beneficial insects 131 

Insects as carriers of disease in 

Man 113, 119, 120 

Insecticides and their uses ...106, 

122, 125 

Isle of Wight — Blue disease ... 133-34 
Italv, control of Mulberrv Scale in 133 



Jerusalem artichoke 26 

Jeyes' fluid 86 

"Jumpers" (Psyllidea) 126 

Juniper 93 



Kainit 115 

KaJcothrips robust us HI 

Knot Root Eelworm 100 



140 



PAGE 

Laclv Birds 131, 132 

Larch Sawfly 130 

Larvae 105 

Late blight of potatoes 73 

symptoms 74 

^spread 74, 75 

Late bliglit of potatoes — precau- 
tions 78 

Late blight and potato tubers 76 

Law against barberry 90 

Layering 29 

Lead, arsenate of ... 108, 109, 110, 111 

Leaf blotch of cucumbers 85 

Leaf-cuttings 32 

Leaf mosaic 17 

Leaves, absorption of water by ... 23 

arrangement of 18 

needle-shaped 23 

pores of 21 

wilting of 22 

structure of 21 

work of 20 

Leather Jackets " 113 

Lecanium yersicae 127 

Leguminous plants 13 

Leigh, quoted 99, 135 

Lepidoptera 107 

Leindosaphes ulmi 127 

Light, efeect of, on plants 5 

Lightning, injury bv 53 

Lily ^...."^ 29 

Limacid^ 101 

Limax maximtis 102 

Lime for Cabbage Root Fly 117 

Lime and Carbolic Acid, remedy 

for Onion Fly ^ 118 

Lime and Salt spraying 125, 126 

Lime and Soot, remedy for Slugs 102 
Lime and Sulphur for "big bud" 104 

Liming of soils 11 

Liming of soil against " Club 

Root " disease 69 

Linnet 98 

Literature bearing upon Economic 

Zoology 135 

Liver of sulphur 84, 86 

Longevity of seeds 42 

. Long-tailed Tit ....129 



Macrosporium solani 84 

Malformations 49 

Mallow, rust 94, 95 

Manure, farmyard 11 

March moth 109 

Martins 129 

Mealy Bug 127 

Mendel, Gregor 46 

Merodon equestris 119 

Michaelmas daisy 3 

Mildew, American gooseberry ... 88 



PAGE 

Mildew of roses 87 

Mildews, treatment of 88 

Mildews of leaves 87 

Millipedes 103, 130 

Mint rust 94 

Mites 103 

Mollusc a 101, 130 

Moorland plants 23 

"Mosaic disease" 51 

Mosquitoes 113 

Moths 107 

Motile spores 63 

Mould on bread and dung 59 

Mucor 59 

Mucor — spores — germination 59 

Mulberry Scale, control of, in 

Italy ^ 133 

Miisca doniestica 119 

Mushrooms and toadstools 58 

Mussel Scale 127 

Mutations 45 

Myriapoda 130 

Mytilaspis iwmorum 127 

Myzus rihis 122, 123 

Xapthalene for Eelworms 100 

for Wire worms 112 

Xarcissus Flies 119 

" Xasturtiums," enemies of 107 

Natural selection 44 

Xematus ribesii 110 

Xesting boxes 130 

Xicotine spraying 112, 116, 126 

Xitrate of soda 14 

Xitrate of soda, remedy for Slugs 

and Snails \ 102 

Nitrate of soda, , remedy for 

"Leather Jackets" 114 

Nitro-bacterine 13 

Nitrogen, importance of 12 

Nosema apis 133-34 

S^ovius cardinalis 132 

Nutrition of fungi 60 

Onion Fly ^117 

Orange Scale and methods of 

eradication in California 132 

Orthoptera 106 

Overwatering 7 

Ovipositor 114, 133 

Ox3'gen, need for 7 

Paraffin, remedy for Cabbage 

Root 117 

Paraffin and Soft Soap Emulsion 

118, 125, 127 

" Paralysis " in Bees 134 

Parasites ' 60 

Parasitic Insects 131, 133 

Parsnips, insect enemies of 115 



HI 



PAGE 



Pea Thrips Ill 

Peas, insect enemies of Ill 

Pear, insect enemies of ... 108, 109, 114 

Pear Midge 114 

Peat, bacterised 13 

Pebrine in silkworms 135 

Peony 3 

Perennials 3 

Phosphoric Acid, importance of ... 14 
Fhytoijhthora compared witli 

Phythium 75 

rhytoyhthora, conidia 74 

disease in Asters 81, 83 
growth from diseased 

tubers ... 77 

hibernation 77 

infestans 73 

initial infections ... 77 
resting spores on 

artificial media 77 

Pieris brassicae and P. rapae 107 

Pine-rust 93 

Pipit 129 

Plant Lice 121 

Plasmodiophora Brassicae 65, 66 

Plasmodiophora compared with 

Pythium 67 

Plover 129 

Plum Aphides 122, 124 125 

Plum, insect enemies of 108, 122 

Pollination 37 

Potash, importance of 14 

Potato 26 

cut for " sets " 27 

disease, resistant A^arieties 79 

disease 73 

grown in the dark 27 

attacked by Wireworms ... 112 
attacked by Millipedes ... 131 



Black Scab or Wart 

disease 70 
Potatoes^ Black Speck or Violet 

Bhizoctonia disease 71 
control of scab diseases 
Brown Scab or Ordinary 

Scab 72 

mechanical irritation 72 

Spongy or Powdery Scab... 72 

storage 76 

Precautions against late blight 

disease 78 

Predaceous Insects 131, 132 

Primrose, Jack-in-the-Green 51 

Pruning 34 

Pruning against big bud " 104 

Pruning against Currant Aphides 125 
Pythium compared with Plasmoclio- 

phora 67 

Puccinia graminis 90 

menthae 94 

Aecidiospores in 94 



PAGE 

Puccinia menthae 

Teleutosijores in 94 

Uredospores in 94 

Pulvinaria vitis 127, 

Pupa 105' 

Puparium 113 

Psylla mali 126 

Pythium compared with Phytoyli- 

thora 75 

Pythium de Baryanum 61 

Life History ... 61, 

Pythium, resting spores 63 

reproduction 62, 63 

Quassia and Soft Soap spraying ... 125 
Quicklime as a fungicide " 69, 70 



Radishes, insect enemies of 116 

Recessive characters 46 

Red currant, layering of 29 

Red Spider 103 

Reproduction, vegetative 25 

Respiration of plants, see breath- 
ing , 

Resting filaments of cercospora 

melonis 85 

Resting spores 63, 68 

Retarded crowns 36 

Rhizoctonia violacea 71 

Rhopalosijihon rihis 122, 123 

Ridging 16 

Ringing 30 

Robin 98 

Rook 98, 99, 114 

Root, absorption by 9 

cuttings 31 

hairs 10 

pressur3 9 

pruning 34 

tipS; sensitiveness of 4 

tubercles 12 

Rose Aphides 122, 123 

Rose mildew 87 

Rotation of crops 15 

Rotation of twining stem 19 

Runners 29 

Russell, E. J 14 

Rust of wheat 90 

Rust on gooseberry 93 

Rust resistant wheats 93 

Rusts 89 

Ruston, A. G 56 

Sandflies 113 

Saproph3^tes 60 

Sawflv 110, 130 

Scale Insects 127 

Enemies of 129, 132, 133 

Scattering of spores 59 



142 



PAGE 

Sell izuiuura lanlycra 122 

Sedge and gooseberry rust 93 

Seed coat ' 42 

Seeds, germination of 42 

maturing of 40 

Seed potatoes 78, 80 

reproduction 3 

Seeds, vitality of 41 

Seedlings " d'^amping off" 61 

Seedlings 44 

Selection, natural 44 

Self-fertile flowers 38 

pollination 37 

sterile flowers 38 

Septoria lycopcrsici 84 

Seven-spot Ladybird 131 

Silkworms, Pebrine in 135 

Siphonophora (Rose Aphides) 122. 123 

Skylark 98 

Sleei^y disease of tomatoes 84 

Slightly diseased tubers, effect of 

planting 78 

Slugs 101, 130 

natural enemies of 103 

Slug, carnivorous 130 

Smoky atmosphere, effect of 54 

Snails 101 

natural enemies of 103 

Sparrow, hedge 129 

house 98 

Sphaerotlieci pannosa 87 

Spongosp)ora Solani 72 

Spores of Cladosporium fiilvum ... 84 

Plasmodiophora 67 

Sporidia from teleutospores 92 

Sports 45 

Spraying of potatoes 79 

with lead arsenate ... 108 
109, 110. Ill 

with nicotine 112 

wdth soft soap 112 

Spring spraying 125 

Soft soap spraying 112. 123 

with quassia 125 

Soil, acidity of 11 

bacteria of 12, 13 

Soils, chemistrv of 12 

Soil disinfectants 83. 86 

'Soil, partial sterilisation of ...14. 64, 

83, 101 

phvsical condition of 10 

spores of fungi ... 63.64. 68, 82 

Soot for Cabbage Root Fly 117 

and Lime for Slugs 102 

for Onion Flv 118 

Starling 98, 99, 103, 114 

Steam, for sterilising soil 14 

Stem Eelworm 99 

function of 18 

Sterilisation of soil 64, 83, 101 

Stocks, insect enemies of 116 



PAGE 

Stomata 21 

Storage of potatoes 76 

Strawberries, Eelworm s attacking 100 
Insect enemy of ... 131 

Straw^berry snail 101 

Subsoil .\ 15 

Sulphate of Potash for Eelworms 100 

Swallow 129 

Sw^edes, Fingers-and-Tues " in ... 65 
Swedes, insect enemies of ... 116, 122 

Sweet Pea 39 

Sweet Peas ciossed 47 

Swift 129 

Swift's arsenate paste 108 

Swimming spore? 63 

Symptoms of wilt in Asters 81 

Syjichytriiim endobioticum 70 

Syrphidae 132 



Teleutospores of Puccinia 

niah'actarion 95 
of wheat rust ... 90, 91 

Tendril 19 

Testacellu 130 

Theobald quoted 102, 114, 125 

Thrips Ill 

Thrush, Mirtle 129 

song 98 

Thysanoptera Ill 

Tipida oJcracca 113 

Tits 98, 129 

Toluene, for sterilising soil 14 

Tomato, leaf rust 83 

pollination of 37 

Tomatoes attacked bv Eelworms 100 
by White Fly... 127 

Transpiration 8. 23 

Transplanting 24 

Traps for Earwigs 107 

for Leather Jackets 114 

for \Yire worms 112 

''Tree Tanglefoot" 109, 125 

Trenching 15 

Trombididte 103 

Tsetse flies 113 

Tubers 4. 26 

Tulip 28 

Turnip Aphis 122 

Fingers-and-Toes " 65 

structure of healthy root 66 
Turnips, insect enemies of ... 116. 122 
Turf Traps for "Leather Jackets " 114 

Twdners 18 

Tylenchiis devastatrix 99 

L^redospores of wheat rust 90, 91 

Variation in plants 44 



M-3 



PAGE 

Variegated leaves 50 

Vegetative fi^^meriT? of fungus ... 58 

V-^^:- ' :i 5. 2c 

Vcriv. 114 

Vine. :^c:._^ . - : . v. lacking 127 

Viola, la;.--:; : 50 

Vitality o: ^cr ir 41 

Voss' Tohacco extract 112 

Wagtail 129 

AA'sr: Di-ea^e of potatoes 70 

V --^1 . 22 

\V-,-.:i^ ^ 2 

WeigcUa. pruning of 35 

W^vi- : uih pine-rust 93 

AVliea: rust 90 

Whi:^ Fly 127 



PAGE 

Wilting 01 leaves 22 

Wi: ter Moth 108 

r potatoes 76 

ying 125. 127 

V .vii ... : 112 

■• False Wirewoims " 131 

Wrhiu:: Washes 127 

Wood Snail 101 

Wood Pigeon 98 

V\'oodlice 103 

Woodnecker 98 

Wrr:ll~- AlViis 122. 124 

W : : ant Scale '. 127 

Wrc- 129 

WiUow Wren 129 



Yellow-hammer 98 



NOW READY. Price i/- net. Postage 2d. 



A POCKET SYNOPSIS 

OF THE 

Families of British Flowering 

Plants 

(Based upon the system of Engler) 

EY 

W. B. GROVE, M.A., 

Lecturer in Bota?iy at the Birmi?igham Alunicipal TecJmical 

School, 



Manchester 
AT THE UNIVERSITY PRESS. 



LONGMANS, GREEN c\: CO., 
London, New York, Bombay, 
etc. 



LIBRARY OF CONGRESS 




0 002 81 1 403 5 # 



